Allow customization of HTTP method

libvmod_curl is quite useful for sending requests to peer or "side" caches and even public CDNs, but it's even _more_ useful if one can control the http protocol "method" used in such requests as this is often a non-standard or defacto mechanism of performing authoritative cache management (i.e. ``PURGE /foo HTTP/1.1``).

This PR adds such capability to libvmod_curl.