API reacts to wrong methods

Job #72050 failed for f90ec879:

Unlike described in the API endpoint documentation, the endpoint /bg/clear listens to the HTTP methods GET, HEAD, PUT, DELETE, OPTIONS, TRACE, and PATCH. It's unclear whether the behavior for such calls is identical to the expected POST-calls. Nevertheless, no other functionality than the one of the the POST-calls is indented.

The API should reject calls with the undocumented methods.