fix($sce): allow IE7 standards mode to pass non-quirks mode test

Changes documentMode test version to 7 in order to support IE 8 in IE 7 standards
mode while still protecting against quirks mode.

documentMode returns the following values:
5 - quirks mode,
7 - IE 7 standards mode,
8 - IE 8 standards mode.

Closes #3633
Closes #3646