ActivityPub requests on routes which do not support activitypub
are now replying with HTTP status code 406 "Not Acceptable".
ActivityPub clients are required by the W3C TR to set the `Accept`
header. If this accept header is detected on an unexpected route,
the whole request will be aborted with the status code above.
This is an additional measure for clients who might not be aware of
having to check the content-type header of the reply.
Ref: https://github.com/w3c/activitypub/issues/432
Changelog: Security
Johann150
624157f03e
