Merge headers

Otherwise the headers from `JSONResponse` are gone and the Content-Type of the response would be `text/html` instead of `application/json; charset=utf-8`. This leads to broken scripts since we set the `nosniff` tag, furthermore this is very bad from a security PoV.
Fix unit test
Merge pull request #12619 from owncloud/use-also-original-hea

Merge headers