http: add Head function for making HTTP HEAD requests

R=rsc
CC=golang-dev
https://golang.org/cl/1581041

src/pkg/http/client.go

@@ -130,7 +130,6 @@ func Get(url string) (r *Response, finalURL string, err os.Error) {
 	return
 }
 
-
 // Post issues a POST to the specified URL.
 //
 // Caller should close r.Body when done reading it.
@@ -154,6 +153,19 @@ func Post(url string, bodyType string, body io.Reader) (r *Response, err os.Erro
 	return send(&req)
 }
 
+// Head issues a HEAD to the specified URL.
+func Head(url string) (r *Response, err os.Error) {
+	var req Request
+	req.Method = "HEAD"
+	if req.URL, err = ParseURL(url); err != nil {
+		return
+	}
+	if r, err = send(&req); err != nil {
+		return
+	}
+	return
+}
+
 type nopCloser struct {
 	io.Reader
 }

src/pkg/http/client_test.go

@@ -28,3 +28,13 @@ func TestClient(t *testing.T) {
 		t.Errorf("Incorrect page body (did not begin with User-agent): %q", s)
 	}
 }
+
+func TestClientHead(t *testing.T) {
+	r, err := Head("http://www.google.com/robots.txt")
+	if err != nil {
+		t.Error(err)
+	}
+	if _, ok := r.Header["Last-Modified"]; !ok {
+		t.Error("Last-Modified header not found.")
+	}
+}