Copter: accept DO_CHANGE_SPEED outside of missions

https://github.com/diydrones/ardupilot/issues/1095

ArduCopter/GCS_Mavlink.pde

@@ -1098,6 +1098,19 @@ void GCS_MAVLINK::handleMessage(mavlink_message_t* msg)
             }
             break;
 
+        case MAV_CMD_DO_CHANGE_SPEED:
+            // param1 : unused
+            // param2 : new speed in m/s
+            // param3 : unused
+            // param4 : unused
+            if (packet.param2 > 0.0f) {
+                wp_nav.set_speed_xy(packet.param2 * 100.0f);
+                result = MAV_RESULT_ACCEPTED;
+            } else {
+                result = MAV_RESULT_FAILED;
+            }
+            break;
+
         case MAV_CMD_MISSION_START:
             if (set_mode(AUTO)) {
                 result = MAV_RESULT_ACCEPTED;