[MusicXML import (DOM and PULL parsers)] Wrong import of InstrumentChange elements

LUbuntu 15.04, commit bf8dc84.

Steps to reproduce:

1. Comment the line "define PULL_PARSER true" in importxml.cpp to switch to the DOM parser.
2. Create a new score with two instruments: Piano and Harpsichord.
3. Put a few notes in order to check the sound later.
4. Add InstrumentChange (Classical Guitar) to the first part, put a few notes.
5. Add InstrumentChange (Voice) to the second part, put a few notes.
6. Export file to MusicXML format (*.xml).
7. Import file to MuseScore.

Expected behaviour: notes sound as they should, we have 2 parts with 2 InstrumentChange elements, 4 entries in Mixer.
Actual behaviour: notes sound not as they should, we have 2 parts with 6 InstrumentChange elements, total 8 entries in Mixer (some instruments duplicated).

Import log:

ChordList::read failed:
importMusicXml(0x8e10970, /home/xxx/Documents/MuseScoreDevelopment/Scores/test.xml)
Validation time elapsed: 125 ms
importMusicXml() file '/home/xxx/Documents/MuseScoreDevelopment/Scores/test.xml' is a valid MusicXML file
MusicXmlInstrList::setInstrument instr '', tick 0/1 (0): element already exists
MusicXmlInstrList::setInstrument instr '', tick 2/1 (3840): element already exists
ImportXml: warning: break on first measure
xmlPart: changed instrument '' at tick 3840 not found in part 'P1'
xmlPart: changed instrument '' at tick 9600 not found in part 'P1'
xmlPart: changed instrument '' at tick 14400 not found in part 'P1'
ImportXml: warning: break on first measure
xmlPart: changed instrument '' at tick 1920 not found in part 'P2'
xmlPart: changed instrument '' at tick 5760 not found in part 'P2'
xmlPart: changed instrument '' at tick 9600 not found in part 'P2'
xmlPart: changed instrument '' at tick 11520 not found in part 'P2'
Parsing time elapsed: 6 ms
importMusicXml() return 0


I know that MusicXML doesn't support InstrumentChange elements, but import/export works for simple scores: for example, for a score with one part with one instrument change.
Exported MusicXML file looks correct, so I think it's import problem.