net/url: normalize scheme to lower case (http not HTTP)

Also document %2f vs / ambiguity in URL.Path.

Fixes #3913.
Fixes #3659.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/7225076

src/pkg/net/url/url.go

@@ -220,6 +220,12 @@ func escape(s string, mode encoding) string {
 //
 //	scheme:opaque[?query][#fragment]
 //
+// Note that the Path field is stored in decoded form: /%47%6f%2f becomes /Go/.
+// A consequence is that it is impossible to tell which slashes in the Path were
+// slashes in the raw URL and which were %2f. This distinction is rarely important,
+// but when it is a client must use other routines to parse the raw URL or construct
+// the parsed URL. For example, an HTTP server can consult req.RequestURI, and
+// an HTTP client can use URL{Opaque: "/Go%2f"} instead of URL{Path: "/Go/"}.
 type URL struct {
 	Scheme   string
 	Opaque   string    // encoded opaque data
@@ -371,6 +377,7 @@ func parse(rawurl string, viaRequest bool) (url *URL, err error) {
 	if url.Scheme, rest, err = getscheme(rawurl); err != nil {
 		goto Error
 	}
+	url.Scheme = strings.ToLower(url.Scheme)
 
 	rest, url.RawQuery = split(rest, '?', true)
 

src/pkg/net/url/url_test.go

@@ -251,6 +251,15 @@ var urltests = []URLTest{
 		},
 		"file:///home/adg/rabbits",
 	},
+	// case-insensitive scheme
+	{
+		"MaIlTo:webmaster@golang.org",
+		&URL{
+			Scheme: "mailto",
+			Opaque: "webmaster@golang.org",
+		},
+		"mailto:webmaster@golang.org",
+	},
 }
 
 // more useful string for debugging than fmt's struct printer