Read-only fields ignored on write if value doesn't change.

When some fields are set to read-only, it becomes very difficult to use the API
because the client has to ensure those fields are removed from the object
before to send it back for editing via PATCH. When new read-only
fields are introduced, it has good chance to break every clients already using
the API.

This fix ignores submitted read-only fields in PATCH if the submitted values
aren't changed compared to the stored document.

Closes #479