Fix update of request headers

[epif4nio]

This is a fix to the issue #459

The update function changes the object but always returns None, which resulted in the value returned by get_request_headers being empty all the time.

Among other things that could happen, the Authorization header was not being sent, resulting in a 401 status.

[justb4]

Sorry, took too long. But thanks for this fix (and your patience)! Indeed dict.update() returns None.