fix: status code for MKCOL on existing resource (#142)

fix: status code for MKCOL on existing resource (#142)

* Fix status code for MKCOL on existing resource

Per https://datatracker.ietf.org/doc/html/rfc4918#section-9.3.1,
MKCOL should return a 405 if the resource already exists.

Impetus for this change:
I am using dufs as a webdav server for [Joplin](https://joplinapp.org/)
which interpreted the previous behavior of returning a 403 as an error,
preventing syncing from working.

* add test

Co-authored-by: sigoden <sigoden@gmail.com>