NbDatepicker not working on IE11

### Issue type

**I'm submitting a ...**

* [x] bug report
* [ ] feature request

### Issue description

**Current behavior:**
The Nebular datepicker component seems not to work on IE11.

**Expected behavior:**
IE11 is a supported browser, so the component should works.

**Steps to reproduce:**
Open http://akveo.com/ngx-admin/pages/forms/datepicker with IE11.
Opening the first datepicker and selecting a date results in no date selected in the form (what happens under the hood in unknown).
Opening the second one and selecting a date results in the component stucked with no possibility to select the end date or to close the picker.
The third one has the same behaviour of the first.

### Other information:
Same issue on project with the following configuration:

OS: Windows
node. 10.15.1
npm: 6.9.0
package.json:

"dependencies": {
"@angular/animations": "^7.2.1",
"@angular/cdk": "^7.2.1",
"@angular/common": "^7.2.1",
"@angular/compiler": "^7.2.1",
"@angular/core": "^7.2.1",
"@angular/forms": "^7.2.1",
"@angular/http": "^7.2.1",
"@angular/platform-browser": "^7.2.1",
"@angular/platform-browser-dynamic": "^7.2.1",
"@angular/router": "^7.2.1",
"@auth0/angular-jwt": "^2.1.0",
"@nebular/auth": "^3.1.0",
"@nebular/bootstrap": "^3.1.0",
"@nebular/date-fns": "^3.1.0",
"@nebular/security": "^3.1.0",
"@nebular/theme": "^3.1.0",
"@ng-bootstrap/ng-bootstrap": "^4.0.1",
"@swimlane/ngx-charts": "^10.0.0",
"@types/video.js": "^7.2.10",
"angular-tag-cloud-module": "^3.3.0",
"angular2-chartjs": "0.4.1",
"chart.js": "^2.7.3",
"classlist.js": "1.1.20150312",
"core-js": "2.5.1",
"date-fns": "^2.0.0-alpha.27",
"echarts": "^4.0.2",
"intl": "1.2.5",
"nebular-icons": "1.1.0",
"ng2-nouislider": "^1.7.13",
"ngx-echarts": "^4.0.1",
"normalize.css": "6.0.0",
"nouislider": "^13.1.1",
"npm": "^6.6.0",
"pace-js": "1.0.2",
"roboto-fontface": "0.8.0",
"rxjs": "6.3.0",
"rxjs-compat": "6.3.0",
"typeface-exo": "0.0.22",
"video.js": "^7.4.1",
"web-animations-js": "2.2.5",
"zone.js": "^0.8.28"
}