FIX handle Mastodon using Content-Type header instead of Accept

When Mastodon fetches the profile URL for an actor, it sets the
ActivityPub `Accept` header,
[as specified by the standard](https://www.w3.org/TR/activitypub/#retrieving-objects).

But then, when it sends Follow activities, it switches to using
[Content-Type header](https://github.com/mastodon/mastodon/blob/main/app/workers/activitypub/delivery_worker.rb#L23).

Not sure why it does that, I thought `Content-Type` was for responses,
not for requests. Anyway, we need to cover that case.