feat(http): added withCredentials support

Taken into account the withCredentials property within the request options:
- added corresponding property in the RequestOptions class
- added corresponding property in the Request class
- handle this property when merging options
- set the withCredentials property on the XHR object when specified

Added a test in the xhr_backend_spec.ts to check that the property is actually
set on the XHR object

Closes https://github.com/angular/http/issues/65

Closes #7281

Closes #7281

modules/@angular/http/src/backends/xhr_backend.ts

@@ -35,7 +35,9 @@ export class XHRConnection implements Connection {
     this.response = new Observable<Response>((responseObserver: Observer<Response>) => {
       let _xhr: XMLHttpRequest = browserXHR.build();
       _xhr.open(RequestMethod[req.method].toUpperCase(), req.url);
-
+      if (isPresent(req.withCredentials)) {
+        _xhr.withCredentials = req.withCredentials;
+      }
       // load event handler
       let onLoad = () => {
         // responseText is the old-school way of retrieving response (supported by IE8 & 9)