Crash when loading release with a tag that contains only whitespace.

If a tag for a loaded release contains only whitespace Picard will crash with the error below. This is easily reproduced by adding e.g. "$set(sometag, )" to the tagger script.

Traceback (most recent call last):
File "/home/phw/devel/musicbrainz/picard/picard/webservice/_init_.py", line 443, in _process_reply
self._handle_reply(reply, request)
File "/home/phw/devel/musicbrainz/picard/picard/webservice/_init_.py", line 430, in _handle_reply
handler(document, reply, error)
File "/home/phw/devel/musicbrainz/picard/picard/album.py", line 250, in _release_request_finished
self._finalize_loading(error)
File "/home/phw/devel/musicbrainz/picard/picard/album.py", line 341, in _finalize_loading
track.metadata.strip_whitespace()
File "/home/phw/devel/musicbrainz/picard/picard/metadata.py", line 384, in strip_whitespace
self.apply_func(str.strip)
File "/home/phw/devel/musicbrainz/picard/picard/metadata.py", line 368, in apply_func
for name, values in self.rawitems():
RuntimeError: dictionary changed size during iteration
zsh: abort (core dumped) ./tagger.py