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/static_request.ts

@@ -59,6 +59,8 @@ export class Request {
   private _body: any;
   /** Type of the request body **/
   private contentType: ContentType;
+  /** Enable use credentials */
+  withCredentials: boolean;
   constructor(requestOptions: RequestArgs) {
     // TODO: assert that url is present
     let url = requestOptions.url;
@@ -81,6 +83,7 @@ export class Request {
     // Defaults to 'omit', consistent with browser
     // TODO(jeffbcross): implement behavior
     this.headers = new Headers(requestOptions.headers);
+    this.withCredentials = requestOptions.withCredentials;
   }