Execute step callback for last step. see #18
Execute step callback for last step. see #18

It will execute step even if `currentTime >= endTime` it will
simply not trigger the loop if tween `didEnded`.

acidhax/shifty,GerHobbelt/shifty,acidhax/shifty,webdev1001/shifty,modulexcite/shifty,modulexcite/shifty,jeremyckahn/shifty,GerHobbelt/shifty,webdev1001/shifty