MIDI instrumentTransposition does not transpose key signature

[Dan Eble]

%% The MIDI output should have key signature of C major, but instead
%% has the key signature of D major.

\version "2.19.80" % the version I'm using
\language "english"

soundingPitch = bf

\score {
  \new Staff \with {
    instrumentTransposition = \soundingPitch
  } <<
    \transpose \soundingPitch c' \fixed c' { \key c \major c e g c' }
  >>

  \layout { }
  \midi { }
}