WP_HTTP: Curl: When using Stream-to-file on servers using mbstring.func_overload ensure that the file is written out correctly. Props DrProtocols. See #25061 for 3.6

Built from https://develop.svn.wordpress.org/branches/3.6@25052


git-svn-id: https://core.svn.wordpress.org/branches/3.6@25039 1a063a9b-81f0-0310-95a4-ce76da25c4cd