[discogs_cover_art] User-agent was not set while retrieving images from
Discogs, and thus only "I <3 Discogs" image was retrieved.
[editing] Sort entities by type because MB server expects type0=url and
type1=work and rejects the other way.
[editing] Fix editing relationship.
The methode edit_relationship now uses the internal _relationship_editor_webservice_action() method, that has been changed to be more generic.
Merge pull request #1 from murdos/master

update to master