Make tween.update more robust

It can now handle negative numbers, nil, 0, and any kind of garbage.

Fixes #1455
Actually test what I think I'm testing
adds bb checks
Merge pull request #1460 from hawkthorne/fix_tween_update

Make tween.update more robust
Merge pull request #1468 from CalebJohn/collide

Checks if bb exists on blocks and Platforms