Fuel: Jackson extensions: responseObject() should return CancellableRequest object

[detouched]

Some responseObject(...) extension methods for Jackson don't expose result of CancellableRequest type, which makes them inconsistent with other helper methods and limits their use cases for no reason (e.g. no way to cancel() or join() request when these shorthand methods used).

Specifically, shorthand methods that accept the triple of _request_, _response_ and _result_ have Unit return type.

[SleeplessByte]

This is my fault for not noticing it in #569. Thank you for bringing this up ánd fixing it. You're great.