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 looks like you're right. This PR might help us: https://github.com/intridea/grape/pull/806