Fix #7641: .animate() should accept "auto" and "" (empty string) values

[louisremi]

This PR fixes a "patchwelcome" bug in 10 short lines of code.
Unit-tests included.

http://bugs.jquery.com/ticket/7641 (was "patchwelcome", I just reopened it)

[timmywil]

I'm a little concerned about this because although this adds some support for these values, they can never be fully supported. @gnarf37 also brought up a good point. css animations do not support these values.

[dmethvin]

If something is "kinda" supported, you can bet we'll get plenty of bug reports for the times when it doesn't work. Can we defer this item until animate is refactored in 1.8? We can mark it for 1.8-discuss in the tracker. Sound good?

[louisremi]

Please bear in mind that the limitations are the same when we animate from "auto" and to "auto".
This patch doesn't introduce new limitations to the API, there will be no new warning to add to the documentation.

I've actually queried the bug tracker with "auto" and "animate" keywords, and I haven't found any bug currently open. I guess this means we haven't much problems with people animating from "auto". We will have equally few problems with people animating to "auto", because the logic is the same.

Regarding CSS3 transitions and animations, nowhere in the spec is written that "auto" shouldn't be supported. It is actually supported in chrome and there is an open and accepted bug for firefox: https://bugzilla.mozilla.org/show_bug.cgi?id=571344

[dmethvin]

I'm closing this pull since it's gone stale but leaving the ticket open for discussion.