Fixes issue #1406; there was a memory leak on IE due to very subtle nuances of XHR on IE.  Turns out the only legal way to "unhook" an onreadystatechanged handler is to set it to a dummy func!

Found by: jmgodbout
Review by: knorton


git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@1260 8db76d5a-ed1c-0410-87a9-c151d255dfc7