Ann. Doc.'s API should update the updated_at attribute

[RGreinacher]

This occurs, since the updated_at attribute is part of annotation document's relevant_attributes method. Dalphi's interface data manager stores the whole document, updates just the payload and sends everything (including all the other, untouched attributes like updated_at) back to the API using HTTP's PATCH. This leads to the updated_at attribute being "updated" with the same value it has had before the API update call => the attribute seems not to be chaged.