Implement application/merge-patch+json elegantly

[bfabio]

The handling of application/merge-patch+json is quite manual, in part because of limitation in the ORM, and probably doesn't implement correctly all the cases, but we can improve it.

Instead of doing what we do now, we should:

1. Get the application/merge-patch+json payload and validate it
2. Apply the merge patch to the current JSON representation of the patched resource (and its relationships), (fe. with https://github.com/evanphx/json-patch)
3. Unserialize the result
4. Have the ORM do the required operation to the database based on the unserialized result