17580 500 when posting to textile with json

Implements the following tickets (the commits also reflect this order):

1. https://community.openproject.org/work_packages/17580
2. https://community.openproject.org/work_packages/17439
3. https://community.openproject.org/work_packages/17440
@myabc With ``content_type :txt, 'text/plain'``set the API still accepts different content types. For me it looks like Grape hasn't implemented a content type check.