REFERRER: Stripping a non-relative URL should return 'none', not 'null'.
REFERRER: Clean up the 'determine' algorithm.

Hixie noted that we do a poor job specifying where we get the URL we're
eventually going to use to generate the referrer[1]. This patch attempts
to explain the logic in less hand-wavey ways that don't just pull URLs
out of thin air.

[1]: http://lists.w3.org/Archives/Public/public-webappsec/2014Jul/0101.html
REFERRER: Requests from unique origins should not generate a referer header.

Hixie noted in [1] that HTML's Fetch algorithm drops the 'referer'
header when making requests from unique origins. Firefox honors this
requirement, Chrome currently doesn't. Filed https://crbug.com/397011
to cover it.

[1]: http://lists.w3.org/Archives/Public/public-webappsec/2014Jul/0101.html