Overriding vanilla prototype methods no longer errors

This is something that I've been aware of for quite some time, but never got
around to fixing; ease.js had stalled until it was revitalized by becoming a
GNU project.
Prototype supertype property proxy fix

This was a nasty bug that I discovered when working on a project at work
probably over a year ago. I had worked around it, but ease.js was largely
stalled at the time; with it revitalized by GNU, it's finally getting fixed.

See test case comments for more information.