[testlib] make a clear distinction between input / output HTTP headers The get_header() / set_header() API is the same as for CubicWebRequestBase : - get_header() returns the **request** header - set_header() / add_header() works on the **response** header FakeRequest adds two additional methods : - get_response_header() returns the **reponse** header - set_request_header() adds a **request** header FakeRequest now uses the `headers_out` ineherited from CubicWebRequestBase. It also uses a `_headers_in` attribute