Use free-form MP4 tag for album gain (#2426)

I'm not sure how this got messed up, but this was:
- Trying to store album gain in the track peak part of the SoundCheck tag!
- Not writing the album gain to the non-SC free-form RG tag!

Together, this led to serious weirdness when writing these fields on AAC
files.

To be clear, we currently *only* support track-level data for SoundCheck. We
also record album-level RG information in non-iTunes tags, but that's
separate. A little googling suggests that SoundCheck now has album-level data,
but supporting that is a separate issue.

beets/mediafile.py

@@ -1941,9 +1941,9 @@ def update(self, dict):
             u'replaygain_album_gain',
             float_places=2, suffix=u' dB'
         ),
-        MP4SoundCheckStorageStyle(
-            '----:com.apple.iTunes:iTunNORM',
-            index=1
+        MP4StorageStyle(
+            '----:com.apple.iTunes:replaygain_album_gain',
+            float_places=2, suffix=' dB'
         ),
         StorageStyle(
             u'REPLAYGAIN_ALBUM_GAIN',

docs/changelog.rst

@@ -33,6 +33,8 @@ Fixes:
   requires version 0.4.2 or later of the ``python-mpd2`` library. :bug:`2405`
 * :doc:`/plugins/mpdstats`: Improve handling of mpd status queries.
 * :doc:`/plugins/badfiles`: Fix Python 3 compatibility.
+* Fix some cases where album-level ReplayGain/SoundCheck metadata would be
+  written to files incorrectly. :bug:`2426`
 
 
 1.4.3 (January 9, 2017)