Better validation of the URL used in core HTTP requests.

Merges [30443] to the 3.8 branch.

Built from https://develop.svn.wordpress.org/branches/3.8@30446


git-svn-id: http://core.svn.wordpress.org/branches/3.8@30441 1a063a9b-81f0-0310-95a4-ce76da25c4cd