Handle FLAC albumart correctly.

Fixing https://github.com/rembo10/headphones/issues/1284

I found out that headphones is corrupting FLAC files when it embeds artwork.

The biggest problem for me is that Subsonic won't play the corrupt files as discussed in this thread: http://forum.subsonic.org/forum/viewtopic.php?f=2&t=9434

The cause of the issue is not Headphones per se, rather a outdated beets library that embeds flac files the wrong way: sampsyo/beets@faa2139