* [bugfix] Fix NegotiateAccept with multi accept
There's a bug in Gin's NegotiateFormat that doesn't handle the presence
of multilpe accept headers. This lifts the code from the PR @tsmethurst
sent a year ago to Gin into our codebase to fix the issue.
* [bugfix] Concat accept header in webfinger
Some implementations bug out when there's multiple accept headers,
including Gin (see 7050112af1ccc935ec542cb41fa8b07f7357539d). But things
seem to work reliably with a single accept header with multiple parts.
Fixes: #1793
e91cabb704