Fixes Issue 3608.  XMLHttpRequest now does "new $wnd.XMLHttpRequest"
rather than a bare "new XMLHttpRequest".  Also, it tolerates libraries
that install a non-function as $wnd.XMLHttpRequest on IE6.


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@7676 8db76d5a-ed1c-0410-87a9-c151d255dfc7