http_request passing headers into webtest through keyword argument.

[KoleS46]

The request method doesn't have headers as a positional argument.
def request(self, url_or_req, status=None, expect_errors=False, **req_params):
https://github.com/Pylons/webtest/blob/master/webtest/app.py#L526
This causes the headers to be not present in the request.
Therefore we need to pass it as a keyword one.