Add Allow header for 405 Method Not Allowed replies.

It is required.

http://www.w3.org/Protocols/rfc2616/rfc2616-sec10.html

"10.4.6 405 Method Not Allowed

The method specified in the Request-Line is not allowed for the resource
identified by the Request-URI. The response MUST include an Allow header
containing a list of valid methods for the requested resource."