Fixes Issue #1421.
HistoryImplIE6 did a $doc.write of the unsanitized historyToken exposing an
XSS vulnerability. This fix adds html entity escaping to the token before
it is written.

Found by: akimpton
Review by: scottb, jgw



git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@1257 8db76d5a-ed1c-0410-87a9-c151d255dfc7