Fix FetchAtomService content type handling

[valerauko]

Issue

Fixes #8773

Changes

• add profile to json+ld in the Accept header as required by the spec
• use headers['Content-type'] instead of mime_type
  The latter strips out the profile part of the content type header, so compliant ActivityPub services don't get handled properly. headers['Content-type'] has the "raw" content type with the profile, so use that for checking.

[Gargron]

This broke remote resource resolving because the "raw" header contains stuff like ; charset=utf-8 so of course none of the comparisons match. Also, application/ld+json is a superset of the one with the profile, so I do not follow why it would matter. If I pass an Accept header without the profile part, the responding server is still free to include the profile part in the response, and I am free to ignore it. There aren't many purely ld+json things out there in the wild for that check to be important.

I missed this bug during review and I am very regretful about it. I need to revert this patch.

[valerauko]

Sorry about the trouble caused.