Fix poster-switching tests for IE8, and fix quote handling for other browsers

I fixed @dmlap's test for IE8.  Along the way, I noticed that these tests were failing in Firefox, because Firefox was inserting quotes around the URL in the background-image CSS property.  During testing of my fix for IE8, I discovered that IE10 does the same thing as Firefox in terms of inserting those quotes.

This behavior does not affect the player's ability to change the poster image in that or any browser (I manually regression-tested switching poster images in FF, Chrome and IE8).

Since this PR incorporates all of David's previous work for poster switching, I've spoken to him about it, and #833 can be closed.