Add helpers to improve the evaluation of requests in FakeControllers

[bernardolins]

Currently, the functions of the :cowboy_req module should be used, for example,:cowboy_req.qs_val/2.

It would be interesting to have some helpers that do the same job but in a simpler way. Something like query_string_value(" token ") or header(" Age ")

[bernardolins]

Closing this because FakeControllers are deprecated.