net/url: make Parse+String round trip magnet URLs

Fixes #20054

Change-Id: I3c660ca0c56cdde2c2ac2f6a666d8531ab5588c5
Reviewed-on: https://go-review.googlesource.com/49050
Run-TryBot: Emmanuel Odeke <emm.odeke@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Joe Tsai <thebrokentoaster@gmail.com>

src/net/url/url.go

@@ -726,7 +726,9 @@ func (u *URL) String() string {
 		buf.WriteString(u.Opaque)
 	} else {
 		if u.Scheme != "" || u.Host != "" || u.User != nil {
-			buf.WriteString("//")
+			if u.Host != "" || u.Path != "" || u.User != nil {
+				buf.WriteString("//")
+			}
 			if ui := u.User; ui != nil {
 				buf.WriteString(ui.String())
 				buf.WriteByte('@')

src/net/url/url_test.go

@@ -568,6 +568,28 @@ var urltests = []URLTest{
 		},
 		"",
 	},
+	// test we can roundtrip magnet url
+	// fix issue https://golang.org/issue/20054
+	{
+		"magnet:?xt=urn:btih:c12fe1c06bba254a9dc9f519b335aa7c1367a88a&dn",
+		&URL{
+			Scheme:   "magnet",
+			Host:     "",
+			Path:     "",
+			RawQuery: "xt=urn:btih:c12fe1c06bba254a9dc9f519b335aa7c1367a88a&dn",
+		},
+		"magnet:?xt=urn:btih:c12fe1c06bba254a9dc9f519b335aa7c1367a88a&dn",
+	},
+	{
+		"mailto:?subject=hi",
+		&URL{
+			Scheme:   "mailto",
+			Host:     "",
+			Path:     "",
+			RawQuery: "subject=hi",
+		},
+		"mailto:?subject=hi",
+	},
 }
 
 // more useful string for debugging than fmt's struct printer