net/url: generate correct Path when hostname empty

Parse("file:///foo") previously returned a URL with Scheme "file"
and Path "///foo". Now it returns a URL with Path "/foo",
such that
        &URL{Scheme: "file", Path: "/foo"}.String() == "file:///foo"

This means that parsing and stringifying the URL "file:/foo"
returns "file:///foo", technically a regression but one that only
affects a corner case.

Fixes issue 4189.

R=bradfitz, rsc
CC=golang-dev
https://codereview.appspot.com/7135051