WP_Filesystem: Ensure that all files are read/written correctly by verifying the return values from fwrite() and using FTP_BINARY mode (ASCII converts line endings as per the spec). See #25237

Built from https://develop.svn.wordpress.org/trunk@25304


git-svn-id: http://core.svn.wordpress.org/trunk@25266 1a063a9b-81f0-0310-95a4-ce76da25c4cd