don't use managed buffer (#4)

midi.cpp

@@ -16,8 +16,8 @@ namespace bluetooth {
     //%
     void midiSendMessage(Buffer data) {
         BluetoothMIDIService* pMidi = getMidi();            
-        ManagedBuffer buf(data);
-        switch(buf.length()) {
+        auto buf = data->payload;
+        switch(data->length) {
             case 1: 
                 pMidi->sendMidiMessage(buf[0]);
                 break;