Fixes #6446 - Forbidding non-json POST/PUT requests in v2

## Tasks
- [x] Fix any broken tests
- [x] Write new tests
@daviddavis, nitpick, add "is" before "unsupported" and some other small edits
```ruby
node(:message) { "Content-Type:#{request.content_type} is unsupported in API v2 for POST and PUT requests. Please use 'Content-Type:application/json'." }
```