Better validation of the URL used in core HTTP requests.

Merges [30443] to the 4.0 branch.

Built from https://develop.svn.wordpress.org/branches/4.0@30444


git-svn-id: http://core.svn.wordpress.org/branches/4.0@30439 1a063a9b-81f0-0310-95a4-ce76da25c4cd