Http post not works with new version

Hello,

I am pretty desperate, I don’t understand why the http post does not work after updated to newer version.
I test both on IOS device.

Works: The code below is with version @angular/http 4.4.3
`public login(username: string, passwd: string): Observable {`
`let headers = new Headers({'Content-Type': 'application/x-www-form-urlencoded'});`
`let options = new RequestOptions({headers: headers, withCredentials: true });`
`let params: URLSearchParams = new URLSearchParams();`
`params.set('target', 'https://xxx');`
`params.set('username', username);`
`params.set('password', passwd);`
`return this.http.post('https://xxx', params, options)`
`.map(this.returnData)`
`.catch(this.handleError);`
}`

`private returnData(res: Response) {`
`if (JSON.stringify(res.text()).indexOf('error') === -1) {`
`return res || {};`
`} else {`
`throw {message: 'Error occured'};`
`}`
`}`

`private handleError (error: any) {`
`return Observable.throw(error.message);`
`}`

Not works: The code below is with newer version @angular/http 5.1.0

`public login(username: string, passwd: string): Observable {`
` const headers = new HttpHeaders();`
` headers.append('Content-Type,', 'application/x-www-form-urlencoded');`
` const options = {headers: headers, withCredentials: true};`
` const body = {`
` target: 'https://xxx',`
` username: username,`
` password: passwd`
`};`
` return this.http.post('https://xxx', body, options)`
` .map(this.returnData)`
` .catch(this.handleError);`
`}`

` private returnData(res: HttpResponse) {`
` if (res.ok) {`
` return res || {};`
` } else {`
` throw {message: 'Error occured'};`
` }`
` }`

` private handleError(error: HttpErrorResponse) {`
` return Observable.throw(error.message);`
` }`

Start with the login method and compare the @angular/http 4.4.3 request against the Angular 5.1.0 request shown in the issue. Reproduce the POST on an iOS device, then determine what changed in the request or response handling and verify the behavior with a working request.

由索引模型根据 Issue 内容生成。