Outgoing Content-Type is not scoped

[warriordog]

According to the spec, the acceptable content types are:

• application/ld+json; profile="https://www.w3.org/ns/activitystreams"
• application/activity+json

The latter type is described as optional, and implementations are not required to support it.
For best results, it's recommended to accept either form but use the first for outgoing requests.

Our implementation currently supports only the second, unscoped type along with these extra types for maximum compatibility:

• application/activity+json
• application/ld+json
• application/json

This works against all tested Fedi implementations, but we should include the profile for better spec-compliance. This may require a refactor of ActivityPubOptions.

See also: https://www.w3.org/TR/activitypub/#client-to-server-interactions

[puzzler995]

I'll pick this one up and work on it