Allows client to specify 0 speed

#### Background story
I have a Logitech rumblepad2 for which I've written [an interface with node-ar-drone](https://github.com/michaelshmitty/drone-gamepad) to control my AR drone 2.0.
The joystick controls pitch and roll, the dpad controls yaw, ascend and descend.
When the joystick is centered, pitch and roll should stop.
When a dpad button is released, yaw, ascend, descend should stop.

At first I used client.stop(), but that stops all movement. I didn't want my yaw movement (dpad) to stop whenever I stopped pitching / rolling (joystick).
Instead I let my code figure out what movement should be stopped specifically and set speed to 0 for that movement.

#### Speed of 0 ignored
Then I noticed client.front(0) was being ignored by the node-ar-drone library. I investigated and wrote this fix with a test. My gamepad now works as expected, yay.