Exchange voices adds extra rests

Steps to reproduce bug
1. Open attached file: "exchange voices.mscz"
2. Edit > Select All
3. Edit > Voices > Exchange Voices 1-2

Expected behavior: Notes should remain the same but voices switch

Actual behavior: Measure 2 gets extra rests which corrupts timing.

MuseScore version: 0.9.6.3 stable

(Operating System: Windows 7)

first reported by jwinter: http://musescore.org/en/node/7873#comment-25254