[Trunk] Solo/Mute not saved

1. Create piano score.
2. Enter notes.
3. In Mixer, select Mute.
4. Save and close score.
5. Re-open score.
6. Open Mixer and play score.

Expected result: Piano should be mute.
Actual result: Piano isn't mute.

Everything else in mixer appears to work.

Using MuseScore 2.0 nightly build (4145) - Mac 10.4.11.