Midi equalizer not working in 2.11.16

[Daniel Johnson]

In the following snippet, you'd expect the oboe to be nine times as loud
as the bassoon, but in fact they are the same volume.

%%% BEGIN
\version "2.11.16"
\score {
  <<
    \new Staff {
      \set Staff.midiInstrument = "oboe"
      \set Staff.midiMinimumVolume = #0.9
      \set Staff.midiMaximumVolume = #0.9
      c''1
    }
    \new Staff {
      \set Staff.midiInstrument = "bassoon"
      \set Staff.midiMinimumVolume = #0.1
      \set Staff.midiMaximumVolume = #0.1
      c'1
    }
  >>
  \midi { }
}
%%% END