mediatypes: add band-aid for quay.io handling

[steveej]

Since recently quay.io appends ; charset=utf-8 in the content-type
header value for HEAD requests on the manifests endpoint. This patch
restores functionality as before the quay.io change.

[lucab]

This PR is unfortunately wrong (and I fear the change on quay.io side too).

Media-types are part of the docker-registry API specifications. The V2S1 types are defined here and they don't have any additional (including charset) parameters.

Additionally, those manifests are explicitly compatible with application/json. JSON is a binary media type. As binary, it doesn't really have an outer charset, and the spec explicitly says that it doesn't have any additional (including charset) parameters.

@thomasmckay @josephschorr any chance to revert the change on quay, and align it back with IETF and docker-registry specs?

[josephschorr]

@lucab We can certainly revert, but we'd have to double check on what Docker is returning for manifests. The problem is the Docker documentation itself shows Content-Type: application/json; charset=utf-8 on some of the API returns, so it isn't clear whether that applies to the manifest type when it itself is application/json. I want to be sure before we go back and potentially break utf-8 encoded manifests.

[lucab]

For reference, the quay.io change was reverted in the meanwhile. It was breaking kubernetes CI too: kubernetes/kubernetes#72863 (comment)