Improve method not allowed handling

[sharpner]

We currently have a private methodNotAllowed handler that gets registered only in the case that a user uses the default httprouter.

We should somehow make sure that one gets registered.
Additionally, it currently has an unnecessary dependency on the whole API object just to set the response Content-Type header. We either have to make sure that one API object always has one Content-Type set for all method not allowed, or that the method not allowed header can understand which Content-Type is relevant for each call.

Maybe we can now even auto plug the router for each framework, since because of the build tag, we now which framework is getting used....