[BUG] Incorrect Content-Type header 'text/plain' set for endpoint /lookup/shells

[de-ich]

Describe the bug
API endpoint "/lookup/shells" returns content with response header Content-Type set to text/plain instead of application/json (even if the request header Accept is explicitly set to application/json.

To Reproduce

1. Run the current version of the server
2. Request the /lookup/shells endpoint, e.g. via swagger.
3. (Optional: Set the requested Content-Type to application/json via the Accept header.)
4. Note that the server responds with Content-Type set to text/plain.

Expected behavior
Correct (default) content type application/json is used.

Screenshots

[de-ich]

I opened this issue in the aaspe repo by accident. Recreated the issue in the server repo: eclipse-aaspe/server#364