ActivityPub representation is served with application/json content type

Describe the bug

ActivityPub representation of a podcast is served with application/json content type. However, some federation clients may reject object if content type is different from standard AP or AS media type (as a security measure). This is the case with my Mitra server that can't fetch podcast by its ID. I observed that Mastodon also can't fetch podcast by ID, although I don't have access to logs there, so this might be a different problem.

Steps to reproduce

Make GET request to podcast ID with Accept: application/ld+json; profile="https://www.w3.org/ns/activitystreams" header.

Expected behavior

The value of content type header is expected to be application/ld+json; profile="https://www.w3.org/ns/activitystreams" or application/activity+json.

Actual behavior

The value is application/json

Context

Sorry, the Castopod instance I tested against has disappeared and I can't find another one :)