fix: remove incorrect accept header for issue or revoke credential

[iFergal]

The issue and revoke credential endpoints return an operation like most endpoints, so they should have an application/json content type.

The problem with the client specifying application/json+cesr (which credentials().get(said) can do, but not by default) is the response body is lost if you raise e.g. a falcon.HttpNotFound exception. So if you tried to issue a credential with an unknown registry you would just see HTTP POST /identifiers/issuer/credentials - 404 Not Found - .

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.63%. Comparing base (f59a9ba) to head (4ec081d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #313      +/-   ##
==========================================
- Coverage   83.64%   83.63%   -0.01%     
==========================================
  Files          48       48              
  Lines        4267     4265       -2     
  Branches     1067     1054      -13     
==========================================
- Hits         3569     3567       -2     
  Misses        668      668              
  Partials       30       30              

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[iFergal]

Not sure why the tests failed on the second commit, passing locally. Seems like a flakey (very rarely) test, should be OK on a re-run. (I can't trigger)