Pass through write_concern from update_one to update correctly

update_one seems to incorrectly always pass write_concern=None, rather than passing through the parameters received.

Updated the method to pass write_concern correctly.

The tests for update and update one don't look like they previously did anything that would correctly check if write_concern was being passed correctly. Updated those tests to expect update to return number of updated documents if w is 1, or None if w is 0.