Each handler is able to receive its own JSON back

To improve RESTfulness and make a step to API freezing, we need to make each
handler able to receive back its own JSON without falling to 500 error.

Implements: blueprint nailgun-rest-same-json

Change-Id: I78d910eac3d7b067a9930000aab6f568214bf2eb
Merge "Each handler is able to receive its own JSON back"