Datagram Unix socket doesn't work

`_do_connect()` fails if `socket_type` is `"udp"` and `path` is specified instead of `host, port`

In this case, a UDP socket is created, and then its `:connect()` method is called, but there is no such method, so `attempt to call method 'connect' (a nil value)` error is thrown.

A simple fix is here: https://github.com/cloudflare/lua-resty-logger-socket/pull/35