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
@hschink I'm surprised Grape doesn't have a mechanism for this built in. Can you not declare an additional `content_type :txt, 'text/plain'` just for this resource?