MIME types right support

[fgalan]

(Originally this was a comment on #947)

Raised in https://github.com/telefonicaid/fiware-orion/pull/958/files#r31917180, right processing of Accept in /v2/entities operations is not part of PR #958 and would be addressed in a next PR (maybe developing an overall solution for all /v2/ operations, which are purely based on JSON).

Request only support Content-Type application/json and (in some operations) text/plain. Other types (e.g. XML) should be rejected (I don't remember right now the HTTP code to use, but I think there is one special code for this).

Payload is returned in application/json, thus Accept must include application/json (note that /, the default Accept if client doesn't include the header, includes application/json). The exception are the operations that also can return text/plain when the client uses options=text. In that case, payload is returned in text/plain, thus Accept must include text/plain.

[fgalan]

Implemented in PR #2323. Moved to the QATestPending milestone for final validation by QA.

[fgalan]

Issues in milestone QATestPending are going to be closed (it is not realistic to think we would have people working on behave testing at the present moment). However, issue is labeles with QATestPending label in the case thinks change and we need to recover it.