Allow relaxed Content-Type for atomic operations

[bkoelman]

This PR permits using a relaxed (simpler) variant of the atomic:operations media type in the Content-Type request/response HTTP header, because OpenAPI client generators often choke on the double quotes in the official media type (producing code that doesn't compile). By default, the Content-Type used for responses is the same as the incoming one, although it can be overruled by sending an Accept HTTP header.

Official header value (existing):
application/vnd.api+json; ext="https://jsonapi.org/ext/atomic"
Relaxed variant (added in this PR):
application/vnd.api+json; ext=atomic-operations

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• N/A: Documentation updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.84%. Comparing base (a8ff4ec) to head (94d7332).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1553      +/-   ##
==========================================
+ Coverage   90.81%   90.84%   +0.02%     
==========================================
  Files         346      346              
  Lines       11063    11097      +34     
  Branches     1815     1821       +6     
==========================================
+ Hits        10047    10081      +34     
  Misses        669      669              
  Partials      347      347              

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