faraday-lwt-unix: handle EPIPE the same way as BADFD is handled
The socket is closed for writing
This fixes an issue when a HTTP/AF + Opium server receives a request,
where the requestor closes the TCP connection before the response has
been sent completely. Note that the server in question ignores SIGPIPE
(which is then translated to an EPIPE): Sys.(set_signal epipe Signal_ignore)