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/test/backends/xhr_backend_spec.ts

@@ -42,6 +42,7 @@ class MockBrowserXHR extends BrowserXhr {
   responseHeaders: string;
   responseURL: string;
   statusText: string;
+  withCredentials: boolean;
 
   constructor() {
     super();
@@ -511,6 +512,26 @@ export function main() {
            existingXHRs[0].dispatchEvent('load');
          }));
 
+      it('should set withCredentials to true when defined in request options for CORS situations',
+         inject([AsyncTestCompleter], (async: AsyncTestCompleter) => {
+           var statusCode = 200;
+           sampleRequest.withCredentials = true;
+           var mockXhr = new MockBrowserXHR();
+           var connection =
+               new XHRConnection(sampleRequest, mockXhr, new ResponseOptions({status: statusCode}));
+           var responseHeaders = `X-Request-URL: http://somedomain.com
+           Foo: Bar`
+
+                                 connection.response.subscribe((res: Response) => {
+                                   expect(res.url).toEqual('http://somedomain.com');
+                                   expect(existingXHRs[0].withCredentials).toBeTruthy();
+                                   async.done();
+                                 });
+
+           existingXHRs[0].setResponseHeaders(responseHeaders);
+           existingXHRs[0].setStatusCode(statusCode);
+           existingXHRs[0].dispatchEvent('load');
+         }));
     });
   });
 }