Refactor API V3 textile specs
Test that API V3 textile detects wrong content type
Stop RACK from parsing requests for API V3

This allows us to handle parsing errors in the API V3.
Textile endpoint renders 400 on invalid content type
Test render plain endpoint for API V3
Implement render plain endpoint for API V3
Test API V3 WP form response on invalid JSON
Respond with a nice 400 if we cannot parse the JSON request
Merge pull request #2275 from opf/fix/17580_500_when_posting_to_textile_with_json

17580 500 when posting to textile with json