Fixes IE's http status code mangling from 204 to 1223

XMLHTTPRequest object in IE will return a status code of 1223 and drops some
response headers if the server returns a HTTP/204.

This patch intercepts the original response in IE6-9 and returns 204 when the
code is 1223.

Fixes issue 5031.

Change-Id: I97b9094ef702cd852cc4d918183b394ffc853c32
Review-Link: https://gwt-review.googlesource.com/#/c/1440/

Review by: skybrian@google.com

git-svn-id: https://google-web-toolkit.googlecode.com/svn/trunk@11449 8db76d5a-ed1c-0410-87a9-c151d255dfc7