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/65Closes#7281Closes#7281