[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