onlyworlds 0.2.0

• Only if unchanged (keel #210). patch(..., if_match=change_seq) and edit_links(..., if_match=...) send If-Match: the write lands only if the element is still at that change_seq. When it has moved, nothing is written and PreconditionFailed is raised (412 precondition_failed), an ApiError whose current is the element as it is now; ApiError.is_precondition_failed names the case. The server never merges. An if_match that is not a whole number raises ValueError before anything is sent. Without if_match, nothing changes. Checked on keel-staging (tests/test_staging_writes.py).