Canonical redirects should only be applied for GET requests.

props c.axelsson.
fixes #27498.
Built from https://develop.svn.wordpress.org/trunk@28958


git-svn-id: http://core.svn.wordpress.org/trunk@28754 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Avoing a PHP warning in media modal if the attached file does not exist.

props kovshenin.
see #24716.
Built from https://develop.svn.wordpress.org/trunk@28959


git-svn-id: http://core.svn.wordpress.org/trunk@28755 1a063a9b-81f0-0310-95a4-ce76da25c4cd