Better validation of the URL used in core HTTP requests.

Merges [30443] to the 3.9 branch.

Built from https://develop.svn.wordpress.org/branches/3.9@30445


git-svn-id: http://core.svn.wordpress.org/branches/3.9@30440 1a063a9b-81f0-0310-95a4-ce76da25c4cd