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/http.ts

@@ -23,7 +23,8 @@ function mergeOptions(defaultOpts: BaseRequestOptions, providedOpts: RequestOpti
       url: providedOpts.url || url,
       search: providedOpts.search,
       headers: providedOpts.headers,
-      body: providedOpts.body
+      body: providedOpts.body,
+      withCredentials: providedOpts.withCredentials
     }));
   }
   if (isPresent(method)) {