flac2mp3: fix setting the tag

We got hold of a _copy_ of the tag, not the tag itself, so we need
to (re)set the tag before saving.

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>
flac2mp3: the default album artist is empty

Signed-off-by: Ferry Huberts <ferry.huberts@pelagic.nl>