Fixed: withCredentials IE fix (#2158) fails with Firefox

Further work had to be done on this issue due to Firefox not liking the IE11 fix.  I've isolated the IE fix for only IE browsers.  CFHTTPRequest now takes an "isIE" flag to decide whether to set withCredentials before (not IE) or after (IE) opening the XMLHTTPRequest.

Refs: #2157
i wonder whether isIE is passed to the right parameter, given the definition of open is changed to CFHTTPRequest.prototype.open = function(/*String*/ aMethod, /*String*/ aURL, /*Boolean*/ isAsynchronous, /*String*/ aUser, /*String*/ aPassword, /*Boolean*/ isIE). it looks to me as if user and password are omitted.