Transpose key from F# to C will end up to D

Mac OS X 10.7.4 https://github.com/musescore/musescore/commit/b2fdfa32af

- create music score
- set first measure key is F#
- transpose by key, "Up" to C, check "Transpose key signatures" and "Use double sharps and flats"

Expected: transpose to C
Actual: transpose to D

in transpose.cpp, line 333
int nKeyType = transposeKey(oKey.accidentalType(), interval);
nKeyType == 2, after function called, should be 0 ? or why we must get interval first then get nKeyType by interval ?