[xhr] Consider setting omit-Origin-header-flag for same origin requests (#31)

Created in response to https://github.com/whatwg/fetch/issues/91#issuecomment-159190323.

As @yutakahirano reported in https://github.com/whatwg/fetch/issues/91#issuecomment-144348029, Chrome (only GET/HEAD) and Firefox (also for POSTs) are not sending the `Origin` header for same origin requests. The background for Chrome (WebKit) is a little complicated (see https://github.com/whatwg/fetch/issues/91#issuecomment-128261982).

As it's been so for long time, we could make XHR an exception (i.e. no Origin for same origin requests).

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Reply to this email directly or view it on GitHub:
https://github.com/whatwg/xhr/issues/31