[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")

declare option 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 ⛔

_{NEW Get contextual insights on your PRs based on Codacy's metrics, along with PR and Jira context, without leaving GitHub. Enable AI reviewer
TIP This summary will be updated as you push new changes.}