Apply CSS shorthand expansion to IE<9 only

Following up on #1886 and #1901, perhaps this is an edge-case that is unlikely to have a significant real-word impact, but faster is better (and a step towards isolating IE8 fixes). :)

@zpao Feel free to do as you want, but perhaps this is a solution for starting small and going from there.

Unsetting shorthand bug is reproducible on IE when document mode is 8 or less (we don't actually support IE<8, but whatever). According to all information I can find (and common sense), this only applies to IE<8 and no other browser. http://bugs.jquery.com/ticket/12385 https://github.com/jamietre/CsQuery/issues/93
Why does this throw? All the info online I can find indicate that it silently does the wrong thing.