defer_javascript should turn off for requests with header XMLHttpRequest

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From amattie@gmail.com:


I think defer_javascript should automatically disable when the request contains 
a "X-Requested-With: XMLHttpRequest" header or a "Origin" header w/ any value. 
The reasoning is that any content placed on the page by XmlHttpRequest will 
have the expectation that the script will run as soon as it's on the page; in 
the current implementation, those script tags don't appear to run at all.

Like I said earlier, defer_javascript looks very promising and we're excited to 
eventually be able to use it. It appears to speed up rendering significantly; I 
wouldn't have bothered with investigating issues otherwise!

Original issue reported on code.google.com by jmara...@google.com on 27 Jul 2012 at 9:50