OAuth 2.0 code grant flow expects xml encoded not json payload

### Issue type

**I'm submitting a ...** (check one with "x")

* [ * ] bug report
* [ ] feature request

### Issue description
Attempting to leverage ngx-admin templates to recover azure acess tokens with
Azure OAuth 2.0 code grant flow. [](url) https://docs.microsoft.com/en-us/azure/active-directory/develop/v1-protocols-oauth-code

**Current behavior:**

Plan to use ngx-admin template to qury microsoft graph api
To do so i need my app to recover two tokens

1. Token 1 : Authorisation token
1. Token2 : Acess token
Currently i can successfully negotiate the first token as code request from endpoint oauth2/authorize?
However when i attempt to post the authorization token to the second endpoint oauth2/token? to recover an acess token Microsoft complains as follows
`{error: "invalid_request",…}
correlation_id
:
"b5cb3397-b008-4bb0-b475-76ea8deab6c7"
error
:
"invalid_request"
error_codes
:
[90014]
error_description
:
"AADSTS90014: The request body must contain the following parameter: 'grant_type'.
↵Trace ID: d74fbb29-bea3-4bca-b663-b09f65713900
↵Correlation ID: b5cb3397-b008-4bb0-b475-76ea8deab6c7
↵Timestamp: 2018-11-02 09:18:53Z"
timestamp
:
"2018-11-02 09:18:53Z"
trace_id
:
"d74fbb29-bea3-4bca-b663-b09f65713900"`

For a reference here is the post request as recovered from network debugging
`POST https://login.microsoftonline.com/xxxxxxxxxxxxxxxxx/oauth2/token? `

```
REQUEST HEADERS

Host: login.microsoftonline.com
User-Agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:61.0) Gecko/20100101 Firefox/61.0
Accept: application/json, text/plain, */*
Accept-Language: en-GB,en;q=0.5
Accept-Encoding: gzip, deflate, br
Referer: http://localhost:4200/pages/azure/callback?code=AQABAAIAAAC5una0EUFgTIF8ElaxtWjTItiXmuKtpz_KYr3JCsaZfylYKyaMy81yTMl--y5xWN7KkkKbMNdbtgyBeqJy1_0cbmiWZ4pmpVuHzHAp2fZwx6EYi6_Zdg1nCiY0v2y_2PD4EZu3iHWGIE2BCXu7IKw66Oz43lZZmB8hn1x8WRZrOnMUwrxDB_jJDviMAS-LMTed7pE6HoTOy6kUE2kMKCTL_qY2dYMPp16dduOuSvdWV-Suo21wyKaZbAJ37WRiizi0HKr0jWnnWv6KbAXw466fpVkTRyAysw_5bMkHNFCSvf5qmcHZnB_wPmNJnBBl_O4P_SiAbS3ubHuUU-KzyNo1cAijvP5aPssWShmbeTyV1MNnEjjjmfNGaOfGXVk6KKkCGecwbR_QbIpAIJ0-qnR06oqAQOgCSDnTQ204TPQyxehI7Useo7yntddQnt6wnxA26tiCfglCPyBwNUuqpzz8fnwtPydpB7N8pS3FrkOAw3rNS442dDwjqV5L2RpdOqD3GClHtPJgzBiWTEkhayS1CXuxNGr5dAZyr_68b_tXkrmW1LrQ82bvjD-PpvP9a84gAA&session_state=f76e0e48-8613-4c2f-9f53-d10ddd3b5548
Content-Type: application/json
Content-Length: 746
```

```
REQUEST BODY

{"grant_type":"authorization_code","code":"xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx-xxxxxxxxxxxxxxxxxxxxx","redirect_uri":"http://localhost:4200/pages/azure/callback","client_id":"xxxxxxxxxxxxxxxxxxxxxxxxxxxx"}
```

Stack overflow post point out that the second endpoint expects payload as
xml-endoded
https://stackoverflow.com/questions/48996804/azure-active-directory-aadsts90014-invalid-request

**Expected behavior:**

Expect to be able to negotiate the second transaction
Is there a option to enable application/x-www-form-urlencoded for the second trasaction
**Steps to reproduce:**

ngx-admin leveraging oauth2

**Related code:**

oauth2.module.ts
```
/**
* @license
* Copyright Akveo. All Rights Reserved.
* Licensed under the MIT License. See License.txt in the project root for license information.
*/

import { NgModule } from '@angular/core';
import { ThemeModule } from '../../../@theme/theme.module';
import { Oauth2RoutingModule, routedComponents } from './oauth2-routing.module';
import { FormsModule } from '@angular/forms';
import { RouterModule } from '@angular/router';
import { HttpClientModule } from '@angular/common/http';

import {
NbAuthModule,
NbOAuth2AuthStrategy,
NbOAuth2ResponseType,
NbAuthOAuth2Token,
NbOAuth2GrantType,
NbAuthJWTToken,
} from '@nebular/auth';

@NgModule({
imports: [
ThemeModule,
FormsModule,
HttpClientModule,
Oauth2RoutingModule,
NbAuthModule.forRoot({
strategies: [
NbOAuth2AuthStrategy.setup({
name: 'azure',
clientId: 'xxxxxxxxxxxxxxxxxxxxxxxxxx',
clientSecret: 'xxxxxxxxxxxxxxxxxxxxxxx',
authorize: {
endpoint: 'https://login.microsoftonline.com/xxxxxxxxxxxxxxxxxxx/oauth2/authorize?',
responseType: NbOAuth2ResponseType.CODE,
scope: 'https://graph.microsoft.com',
redirectUri: 'http://localhost:4200/pages/azure/callback',
//params: {'resource' : 'https://graph.microsoft.com'}
},
token: {
endpoint: 'https://login.microsoftonline.com/1d2d8da4-2bcf-4021-a2c2-55cf9b00db5b/oauth2/token?',
redirectUri: 'http://localhost:4200/pages/azure/callback',
class: NbAuthJWTToken, // NbAuthOAuth2Token,
},

redirect: {
success: '/pages/azure',
},
},),
],
}),
],
declarations: [
...routedComponents,
],
})
export class NbOAuth2Module {
}

```
oauth2-login.component.ts
```
/**
* @license
* Copyright Akveo. All Rights Reserved.
* Licensed under the MIT License. See License.txt in the project root for license information.
*/

import { NbMenuService } from '@nebular/theme';
import { Component, OnDestroy } from '@angular/core';
import { NbAuthOAuth2Token, NbAuthResult, NbAuthService } from '@nebular/auth';
import { takeWhile } from 'rxjs/operators';

@Component({
selector: 'ngx-oauth2-login',
styleUrls: ['./oauth2-login.component.scss'],
templateUrl: './oauth2-login.component.html',
})

export class NbOAuth2LoginComponent implements OnDestroy {

token: NbAuthOAuth2Token;

alive = true;

constructor(private authService: NbAuthService) {
this.authService.onTokenChange()
.pipe(takeWhile(() => this.alive))
.subscribe((token: NbAuthOAuth2Token) => {
this.token = null;
if (token && token.isValid()) {
this.token = token;
}
});
}

login() {
this.authService.authenticate('azure')
.pipe(takeWhile(() => this.alive))
.subscribe((authResult: NbAuthResult) => {
});
}

logout() {
this.authService.logout('azure')
.pipe(takeWhile(() => this.alive))
.subscribe((authResult: NbAuthResult) => {
});
}

ngOnDestroy(): void {
this.alive = false;
}
}

```
oauth2-callback.component.ts
```
/**
* @license
* Copyright Akveo. All Rights Reserved.
* Licensed under the MIT License. See License.txt in the project root for license information.
*/

import { NbMenuService } from '@nebular/theme';
import { Component, OnDestroy } from '@angular/core';
import { NbAuthResult, NbAuthService } from '@nebular/auth';
import { Router } from '@angular/router';
import { takeWhile } from 'rxjs/operators';

@Component({
selector: 'ngx-oauth2-callback',
styleUrls: ['./oauth2-callback.component.scss'],
templateUrl: './oauth2-callback.component.html',
})

export class NbOAuth2CallbackComponent implements OnDestroy {

alive = true;

constructor(private authService: NbAuthService, private router: Router) {
this.authService.authenticate('azure')
.pipe(takeWhile(() => this.alive))
.subscribe((authResult: NbAuthResult) => {
if (authResult.isSuccess() && authResult.getRedirect()) {
this.router.navigateByUrl(authResult.getRedirect());
}
});
}

ngOnDestroy(): void {
this.alive = false;
}
}

```

### Other information:

**npm, node, OS, Browser**
```

```

**Angular, Nebular**
```
"name": "ngx-admin",
"version": "2.3.0",
"license": "MIT",
"repository": {
"type": "git",
"url": "git+https://github.com/akveo/ngx-admin.git"
},
"bugs": {
"url": "https://github.com/akveo/ngx-admin/issues"
},
"browserslist": [
"> 1%",
"last 2 versions",
"IE 11"
],
"scripts": {
"ng": "ng",
"conventional-changelog": "conventional-changelog",
"start": "ng serve",
"build": "ng build",
"build:prod": "npm run build -- --prod --aot",
"test": "ng test",
"test:coverage": "rimraf coverage && npm run test -- --code-coverage",
"lint": "ng lint",
"lint:fix": "ng lint ngx-admin-demo --fix",
"lint:styles": "stylelint ./src/**/*.scss",
"lint:ci": "npm run lint && npm run lint:styles",
"pree2e": "webdriver-manager update --standalone false --gecko false",
"e2e": "ng e2e",
"docs": "compodoc -p src/tsconfig.app.json -d docs",
"docs:serve": "compodoc -p src/tsconfig.app.json -d docs -s",
"prepush": "npm run lint:ci",
"release:changelog": "npm run conventional-changelog -- -p angular -i CHANGELOG.md -s"
},
"dependencies": {
"@agm/core": "1.0.0-beta.2",
"@angular/animations": "6.0.0",
"@angular/common": "6.0.0",
"@angular/compiler": "6.0.0",
"@angular/core": "6.0.0",
"@angular/forms": "6.0.0",
"@angular/http": "6.0.0",
"@angular/platform-browser": "6.0.0",
"@angular/platform-browser-dynamic": "6.0.0",
"@angular/router": "6.0.0",
"@asymmetrik/ngx-leaflet": "3.0.1",
"@nebular/auth": "2.0.0-rc.9",
"@nebular/security": "2.0.0-rc.9",
"@nebular/theme": "2.0.0-rc.9",
"@ng-bootstrap/ng-bootstrap": "1.0.0",
"@swimlane/ngx-charts": "7.0.1",
"angular-datatables": "^6.0.0",
"angular-tree-component": "7.2.0",
"angular2-chartjs": "0.4.1",
"angular2-toaster": "6.0.0",
"bootstrap": "4.0.0",
"chart.js": "2.7.1",
"ckeditor": "4.7.3",
"classlist.js": "1.1.20150312",
"core-js": "2.5.1",
"datatables.net": "^1.10.19",
"datatables.net-bs4": "^1.10.19",
"datatables.net-dt": "^1.10.19",
"echarts": "^4.0.2",
"font-awesome": "4.7.0",
"intl": "1.2.5",
"ionicons": "2.0.1",
"jquery": "^3.3.1",
"leaflet": "1.2.0",
"nebular-icons": "1.0.8",
"ng2-ckeditor": "1.1.13",
"ng2-smart-table": "1.2.2",
"ngx-echarts": "2.0.1",
"normalize.css": "6.0.0",
"pace-js": "1.0.2",
"primeicons": "^1.0.0-beta.9",
"primeng": "^6.0.0",
"roboto-fontface": "0.8.0",
"rxjs": "^6.1.0",
"rxjs-compat": "^6.1.0",
"socicon": "3.0.5",
"tinymce": "4.5.7",
"typeface-exo": "0.0.22",
"web-animations-js": "2.2.5",
"zone.js": "^0.8.26"
},
"devDependencies": {
"@angular-devkit/build-angular": "~0.6.0",
"@angular/cli": "6.0.0",
"@angular/compiler-cli": "6.0.0",
"@angular/language-service": "6.0.0",
"@compodoc/compodoc": "1.0.1",
"@types/d3-color": "1.0.5",
"@types/datatables.net": "^1.10.12",
"@types/googlemaps": "3.30.4",
"@types/jasmine": "2.5.54",
"@types/jasminewd2": "2.0.3",
"@types/jquery": "^3.3.4",
"@types/leaflet": "1.2.3",
"@types/node": "6.0.90",
"codelyzer": "4.0.2",
"conventional-changelog-cli": "1.3.4",
"husky": "0.13.3",
"jasmine-core": "2.6.4",
"jasmine-spec-reporter": "4.1.1",
"karma": "1.7.1",
"karma-chrome-launcher": "2.1.1",
"karma-cli": "1.0.1",
"karma-coverage-istanbul-reporter": "1.3.0",
"karma-jasmine": "1.1.0",
"karma-jasmine-html-reporter": "0.2.2",
"npm-run-all": "4.0.2",
"protractor": "5.1.2",
"rimraf": "2.6.1",
"stylelint": "7.13.0",
"ts-node": "3.2.2",
"tslint": "5.7.0",
"tslint-language-service": "0.9.6",
"typescript": "2.7.2"
}
```