Fix World.step reverting to fixed time step with 0 dt

Calling World.step with a `timeSinceLastCalled` of 0 would cause it to revert
to the fixed time step behaviour. This was due to the default value of
`timeSinceLastCalled` being 0. This change checks for `undefined`, as opposed
to 0.

See issue schteppe#334 for more details.

src/world/World.js

@@ -501,9 +501,8 @@ var step_mg = vec2.create(),
  */
 World.prototype.step = function(dt,timeSinceLastCalled,maxSubSteps){
  maxSubSteps = maxSubSteps || 10;
- timeSinceLastCalled = timeSinceLastCalled || 0;
 
- if(timeSinceLastCalled === 0){ // Fixed, simple stepping
+ if(timeSinceLastCalled === undefined){ // Fixed, simple stepping
 
  this.internalStep(dt);