amazoncovers can't find covers for tracks with multiple 'artist' tags

Test case:

1. Enable amazoncovers.
2. Make a track with two artist tags. (Should also work with multiple album tags.) Exaile normally displays such tags as "Tag 1 / Tag 2".
3. Play it.

Result:

Track plays, but the following error happens while trying to search for covers.

  File "/exaile/xlgui/cover.py", line 434, in on_playback_start
    update_track=True)
  File "/exaile/xl/cover.py", line 358, in get_cover
    covers = self.find_covers(track, limit=1)
  File "/exaile/xl/cover.py", line 398, in find_covers
    c = method.find_covers(track, limit)
  File "/exaile/data/plugins/amazoncovers/__init__.py", line 38, in find_covers
    (artist, album), limit)
  File "/exaile/data/plugins/amazoncovers/__init__.py", line 51, in search_covers
    ResponseGroup="ItemAttributes,Images")
  File "/exaile/data/plugins/amazoncovers/_ecs.py", line 341, in ItemSearch
    return pagedIterator(XMLItemSearch, argv, "ItemPage", 'Items', plugins)
  File "/exaile/data/plugins/amazoncovers/_ecs.py", line 218, in __init__
    dom = self.__search(** self.__arguments)
  File "/exaile/data/plugins/amazoncovers/_ecs.py", line 350, in XMLItemSearch
    return query(buildRequest(argv))
  File "/exaile/data/plugins/amazoncovers/_ecs.py", line 171, in query
    dom = minidom.parse(usock)
  File "/usr/lib/python2.6/xml/dom/minidom.py", line 1918, in parse
    return expatbuilder.parse(file)
  File "/usr/lib/python2.6/xml/dom/expatbuilder.py", line 928, in parse
    result = builder.parseFile(file)
  File "/usr/lib/python2.6/xml/dom/expatbuilder.py", line 207, in parseFile
    parser.Parse(buffer, 0)
ExpatError: not well-formed (invalid token): line 1, column 651

Reason:

1. amazoncovers calls Track.get_album_tuple to get an (artist, album) tuple.
2. get_album_tuple calls metadata.j to join multiple tags with the same name.
3. metadata.j joins them using NUL.
4. minidom explodes while parsing Amazon's reply containing the original NUL.

Credits: Steve Dodier reported a symptom of the bug in #exaile.