drag event without move and zero deltas

I've added this case for the simulation of the drag event:
`element.simulate("drag", { moves: 0, dx: 0, dy: 0 });`
Using Snap SVG I need to simulate in tests a drag without moves. It will be fired only *mouseup* and *mousedown*.
Only when *dx* and *dy* equal zero I will not assign the default of *3* moves. I have also increased the test of the *drag* including expectations for the *mouseup* and *mousedown*.