net/url: fix regression when serializing relative URLs

Only add a slash to path if it's a separator between
a host and path.

Fixes #6609

R=golang-dev, dsymonds, r
CC=golang-dev
https://golang.org/cl/14815043

src/pkg/net/url/url.go

@@ -459,7 +459,7 @@ func (u *URL) String() string {
 				buf.WriteString(h)
 			}
 		}
-		if u.Path != "" && u.Path[0] != '/' {
+		if u.Path != "" && u.Path[0] != '/' && u.Host != "" {
 			buf.WriteByte('/')
 		}
 		buf.WriteString(escape(u.Path, encodePath))

src/pkg/net/url/url_test.go

@@ -260,6 +260,14 @@ var urltests = []URLTest{
 		},
 		"mailto:webmaster@golang.org",
 	},
+	// Relative path
+	{
+		"a/b/c",
+		&URL{
+			Path: "a/b/c",
+		},
+		"a/b/c",
+	},
 }
 
 // more useful string for debugging than fmt's struct printer