Added expected_data parameter to register_uri

This checks that POST payload received matches the one expected. This is very helpful for improving robustness of some mocked tests, when creating or updating objects, it is very helpful to check data sent to the endpoint was correct, otherwise a bug could be in between and there is no easy way to catch it.

Currently you can access last request's body as you know, but doing it this way is more elegant and easy to follow, as everything is gathered together in the same mock registration. 

Thanks, cheers,
Miguel