Remove handling of zero property-specific keyframes

This case should never occur.
Add test for type of Animation.effect when created with a keyframe dictionary
Handle null property values in keyframes

Also add a test that invalid property values are ignored
Don't add composite or neutral as CSS properties of KeyframeInternal

Also add a couple of asserts for compositing.
Merge pull request #339 from steveblock/keyframeFixes

A few minor tweaks and bug fixes for KeyframeAnimationEffect