Revert "Element.classNames() must not be cached"

This reverts commit 33b9ed4d28cd3eb1ea0cfbcf009cb18f5a0594ce.
Revert "Revert "Element.classNames() must not be cached""

This reverts commit 0e90285bdf51d02b9949f3d5b33b30f59972a7b7.
Test for Issue#478 (class names must not be cached)