yui/yui3 — pull request #225 by clarle (2012-08-18 21:45:17+00:00)
Title: Remove 'X-Requested-With' header for cross-domain requests 

For cross-domain requests using CORS, setting the `X-Requested-By` header leads to a pre-flighting of the request, where an HTTP OPTIONS request header is sent to the server to check if the request is safe to execute.

By removing this header, the cross-domain XHR is considered a simple request, and this pre-flighting is not necessary.

There's more information on MDN:

https://developer.mozilla.org/en-US/docs/HTTP_access_control#Preflighted_requests

And see the similar jQuery ticket, which was accepted:

http://bugs.jquery.com/ticket/4601