[6.x.x] Add media type to writeResultJSON response header

[adamretter]

Backport of #212

RESTServer#writeResultJSON Ignores the XQuery Serialization output media type setting and does not set the Content Type correctly in the HTTP Response, this PR fixes that, and enables support for:

util:declare-option("output:media-type", "application/json")

declareoption output:media-type"application/json";

This PR also adds tests for RESTService to make sure it returns the correct Content-Type headers and prioritize it over the request Accept headers.

[codacy-production]

Not up to standards ⛔

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