Use uri.Path instead of uri.Host for UNIX domain socket URLs

[webconn]

This patch allows to use proper UNIX socket URIs such as unix:///var/run/socket.sock. This format is OK with RFC3986 and used in Docker, for example.

Current implementation makes possible only relative paths (like unix://socket.sock) and it looks like it does so by accident.

Example of how it is parsed by url.Parse(): https://play.golang.org/p/OajK5IJX2F9

package main

import (
	"fmt"
	"net/url"
)

func main() {
	val, err := url.Parse("unix:///var/run/socket.sock")
	fmt.Printf("%#v %v", val, err)
}

&url.URL{Scheme:"unix", Opaque:"", User:(*url.Userinfo)(nil), Host:"", Path:"/var/run/socket.sock", RawPath:"", ForceQuery:false, RawQuery:"", Fragment:"", RawFragment:""} <nil>

[MattBrittan]

Unfortunately it will not be possible to merge this until you sign the ECA (see the contributing section of the readme).

This is not a feature I have used (and I don't really know all that much about the UNIX socket interface) but I am concerned that this may break existing uses (e.g. the example you gave unix://socket.sock). I may be wrong but assume whoever added this functionality had a use for it the way its written; would something like the following work (without breaking any existing usage):

var conn net.Conn
var err error
if len(uri.Host) > 0 {
    conn, err = net.DialTimeout("unix", uri.Host, timeout)
else {
    conn, err = net.DialTimeout("unix", uri.Path, timeout)
}

[webconn]

Good point, added uri.Host check for compatibility. Also I signed the ECA just now, may require triggering checks.

[MattBrittan]

Looks good to me; thanks for your contribution.