Drum parts have the same MIDI port

Ubuntu 15.04, commit d8321d9.

Steps to reproduce:
1. Create a score with 2 drum parts.
2. Export to MusicXML.
3. Open exported file in text editor.

Expected behaviour: each part has it's own MIDI port: 1 and 2.
Actual behaviour: parts have the same MIDI port: 1.

Two parts shouldn't have the same MIDI port and channel.
The problem is not in MusicXML export, but in Score::rebuildMidiMapping().