This prevents jQuery from caching a prefixed property name if provided
directly by the user, e.g. the following code:

	elem.css( "msTransform", "translate(5px, 2px)" );

should not prevent one from from later setting the transition directly:

	elem.css( "transform", "translate(5px, 2px)" );

on a browser not understanding the unprefixed version which is the case
for Safari 8 & transform.

(cherry-picked from d471842b3e3af83c9a1be06b5d16f75bfa96af8c)

Fixes gh-2015
Closes gh-2298