WebResponse: Implement statusHeader() using the new HttpStatus::header()

WebResponse: Implement statusHeader() using the new HttpStatus::header()

* Convert existing use of WebResponse::header() for HTTP status headers
  to use this new statusHeader() method.

* Extend unit test forFauxResponse.

I'm not calling HttpStatus::header directly in code. We keep the abstraction
layer of WebResponse so that responses can continue to be mocked/fauxed without
affecting the outer HTTP response.

Change-Id: I8a536e16659fa88b54cffa1457efb889efa5fcd6