Merge application response headers

- Compose the application response into the SendApiProblemResponseListener
when
 it is an HTTP response.
- When calling `sendHeaders()`, merge in the application response headers into
 the ApiProblemResponse headers to ensure things such as CORS support will
 function.

This should address:

- zfcampus/zf-rest#17
- zfcampus/zf-apigility#20
- zfcampus/zf-apigility#26

(and will actually require that they be reverted)