net/http: clarify that Request.Host may contain a port number

Fixes #4172

R=golang-dev, dsymonds
CC=golang-dev
https://golang.org/cl/6872055

src/pkg/net/http/request.go

@@ -124,6 +124,7 @@ type Request struct {
 	// The host on which the URL is sought.
 	// Per RFC 2616, this is either the value of the Host: header
 	// or the host name given in the URL itself.
+	// It may be of the form "host:port".
 	Host string
 
 	// Form contains the parsed form data, including both the URL

src/pkg/net/http/request_test.go

@@ -228,6 +228,16 @@ func TestReadRequestErrors(t *testing.T) {
 	}
 }
 
+func TestNewRequestHost(t *testing.T) {
+	req, err := NewRequest("GET", "http://localhost:1234/", nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if req.Host != "localhost:1234" {
+		t.Errorf("Host = %q; want localhost:1234", req.Host)
+	}
+}
+
 func testMissingFile(t *testing.T, req *Request) {
 	f, fh, err := req.FormFile("missing")
 	if f != nil {

src/pkg/net/url/url.go

@@ -224,7 +224,7 @@ type URL struct {
 	Scheme   string
 	Opaque   string    // encoded opaque data
 	User     *Userinfo // username and password information
-	Host     string
+	Host     string    // host or host:port
 	Path     string
 	RawQuery string // encoded query values, without '?'
 	Fragment string // fragment for references, without '#'