[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

# Run all scenarii found in windmill directory
from os.path import join, dirname
from cubicweb.devtools.cwwindmill import (CubicWebWindmillUseCase,
                                          unittest_main)

class CubicWebWindmillUseCase(CubicWebWindmillUseCase):
    #test_dir = join(dirname(__file__), "windmill/test_edit_relation.py")
    pass


if __name__ == '__main__':
    unittest_main()