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/interfaces.ts

@@ -30,6 +30,7 @@ export interface RequestOptionsArgs {
   search?: string | URLSearchParams;
   headers?: Headers;
   body?: any;
+  withCredentials?: boolean;
 }
 
 /**