Multisite: Treat 'www' as a special subdomain, reversing 3.9 regression.

Merges [28280] to the 3.9 branch.

props jeremyfelt.
fixes #27927.


git-svn-id: https://develop.svn.wordpress.org/branches/3.9@28281 602fd350-edb4-49c9-b593-d223f7449a82
3.9.1-RC1


git-svn-id: https://develop.svn.wordpress.org/branches/3.9@28282 602fd350-edb4-49c9-b593-d223f7449a82