net/url: prepend slash to path in String()

Previously if a path was set manually without a leading /, String()
would not insert the slash when writing its output. This would lead
to situations where a URL that should be http://www.google.com/search
is output as http://www.google.comsearch

Fixes #5927.

R=golang-dev, bradfitz, rsc, 0xjnml
CC=golang-dev
https://golang.org/cl/11698045

src/pkg/net/url/url.go

@@ -459,6 +459,9 @@ func (u *URL) String() string {
 				buf.WriteString(h)
 			}
 		}
+		if u.Path != "" && u.Path[0] != '/' {
+			buf.WriteByte('/')
+		}
 		buf.WriteString(escape(u.Path, encodePath))
 	}
 	if u.RawQuery != "" {

src/pkg/net/url/url_test.go

@@ -372,6 +372,22 @@ func DoTestString(t *testing.T, parse func(string) (*URL, error), name string, t
 
 func TestURLString(t *testing.T) {
 	DoTestString(t, Parse, "Parse", urltests)
+
+	// no leading slash on path should prepend
+	// slash on String() call
+	noslash := URLTest{
+		"http://www.google.com/search",
+		&URL{
+			Scheme: "http",
+			Host:   "www.google.com",
+			Path:   "search",
+		},
+		"",
+	}
+	s := noslash.out.String()
+	if s != noslash.in {
+		t.Errorf("Expected %s; go %s", noslash.in, s)
+	}
 }
 
 type EscapeTest struct {