Null gateway

Related with #1489 

A Null object never fails, and returns "safe" values with no side-effects.

In this case I think it was fine returning "real" responses instead of creating a "NullResponse" object, since responses are treated as "data" - they don't change the state of anything by themselves.
Can we think of a better distinguishing comment for `BogusGateway`? Maybe something like "Allows triggering specific responses"?