nginx: Add missing WebDAV methods support (PROPFIND & OPTIONS)

nginx: Add missing WebDAV methods support (PROPFIND & OPTIONS)

Nginx provide WebDAV methods PUT, DELETE, MKCOL, COPY, and MOVE with
http_dav_module. But most WebDAV clients that require additional WebDAV
methods (PROPFIND & OPTIONS) to operate. Add missing methods support
with Arutyunyan Roman (arutyunyan.roman@gmail.com)'s nginx-dav-ext-module.
(see: http://nginx.org/en/docs/http/ngx_http_dav_module.html
https://github.com/arut/nginx-dav-ext-module)

Example config:
	location / {
		dav_methods PUT DELETE MKCOL COPY MOVE;
		dav_ext_methods PROPFIND OPTIONS;

		root /var/root/;
	}

Signed-off-by: Ruixi Zhou <zhouruixi@gmail.com>