Return ApiProblemResponse from RestController

- Per zfcampus/zf-api-problem#3 this patch modifies the RestController
  to no longer return ApiProblemModel instances, but rather
  ApiProblemResponse instances. This allows short-circuiting much
  earlier, and eliminates the need for the ApiProblem renderer for
  REST services.
- Anywhere a check for ApiProblem existed, an additional check for
  ApiProblemResponse was added as well.
- In testing, discovered issues with how the Allow header now works in
  the develop branch of zf2, and updated the code to utilize it better.
  These changes were made as I was unsure if they were a result of the
  updates to ApiProblem or not.