XML-RPC: Send 405 Method Not Allowed for GET requests.

props maxcutler.
fixes #20986.

Built from https://develop.svn.wordpress.org/trunk@31004


git-svn-id: http://core.svn.wordpress.org/trunk@30985 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Use proper hash notation for `wp_dropdown_categories()` docblock.

See #30306.
Built from https://develop.svn.wordpress.org/trunk@31005


git-svn-id: http://core.svn.wordpress.org/trunk@30986 1a063a9b-81f0-0310-95a4-ce76da25c4cd