Set Content-Type application/vnd.pub.v2+json

[jonasfj]

The client was originally designed to send Accept: application/vnd.pub.v2+json, to support multiple versions of the API.

I've recently documented this in:
https://github.com/dart-lang/pub/blob/master/doc/repository-spec-v2.md

We should probably start responding Content-Type: application/vnd.pub.v2+json to requests which carry Accept: application/vnd.pub.v2+json.

We might have to interpret no accept header or application/json accept header as to mean return Content-Type: application/json with the version 2 format. We could likely do some logging and figure out of this is necessary and then document it in the API docs.

But just returning Content-Type: application/vnd.pub.v2+json to requests with Accept: application/vnd.pub.v2+json is a good start. It means clients can start to check the Content-Type.