Improve bad handshake error text

Change the error text for bad handshake errors from

    websocket: not a websocket handshake:

to:

    websocket: the client is not using the websocket protocol:

The new text should be more helpful to developers who do not know or
understand the details of the protocol.

Test for bad handshake before other request errors.

client_server_test.go

@@ -398,9 +398,17 @@ func TestBadMethod(t *testing.T) {
 	}))
 	defer s.Close()
 
-	resp, err := http.PostForm(s.URL, url.Values{})
+	req, err := http.NewRequest("POST", s.URL, strings.NewReader(""))
 	if err != nil {
-		t.Fatalf("PostForm returned error %v", err)
+		t.Fatalf("NewRequest returned error %v", err)
+	}
+	req.Header.Set("Connection", "upgrade")
+	req.Header.Set("Upgrade", "websocket")
+	req.Header.Set("Sec-Websocket-Version", "13")
+
+	resp, err := http.DefaultClient.Do(req)
+	if err != nil {
+		t.Fatalf("Do returned error %v", err)
 	}
 	resp.Body.Close()
 	if resp.StatusCode != http.StatusMethodNotAllowed {

server.go

@@ -104,26 +104,28 @@ func (u *Upgrader) selectSubprotocol(r *http.Request, responseHeader http.Header
 // If the upgrade fails, then Upgrade replies to the client with an HTTP error
 // response.
 func (u *Upgrader) Upgrade(w http.ResponseWriter, r *http.Request, responseHeader http.Header) (*Conn, error) {
-	if r.Method != "GET" {
-		return u.returnError(w, r, http.StatusMethodNotAllowed, "websocket: not a websocket handshake: request method is not GET")
-	}
-
-	if _, ok := responseHeader["Sec-Websocket-Extensions"]; ok {
-		return u.returnError(w, r, http.StatusInternalServerError, "websocket: application specific 'Sec-Websocket-Extensions' headers are unsupported")
-	}
+	const badHandshake = "websocket: the client is not using the websocket protocol: "
 
 	if !tokenListContainsValue(r.Header, "Connection", "upgrade") {
-		return u.returnError(w, r, http.StatusBadRequest, "websocket: not a websocket handshake: 'upgrade' token not found in 'Connection' header")
+		return u.returnError(w, r, http.StatusBadRequest, badHandshake+"'upgrade' token not found in 'Connection' header")
 	}
 
 	if !tokenListContainsValue(r.Header, "Upgrade", "websocket") {
-		return u.returnError(w, r, http.StatusBadRequest, "websocket: not a websocket handshake: 'websocket' token not found in 'Upgrade' header")
+		return u.returnError(w, r, http.StatusBadRequest, badHandshake+"'websocket' token not found in 'Upgrade' header")
+	}
+
+	if r.Method != "GET" {
+		return u.returnError(w, r, http.StatusMethodNotAllowed, badHandshake+"request method is not GET")
 	}
 
 	if !tokenListContainsValue(r.Header, "Sec-Websocket-Version", "13") {
 		return u.returnError(w, r, http.StatusBadRequest, "websocket: unsupported version: 13 not found in 'Sec-Websocket-Version' header")
 	}
 
+	if _, ok := responseHeader["Sec-Websocket-Extensions"]; ok {
+		return u.returnError(w, r, http.StatusInternalServerError, "websocket: application specific 'Sec-Websocket-Extensions' headers are unsupported")
+	}
+
 	checkOrigin := u.CheckOrigin
 	if checkOrigin == nil {
 		checkOrigin = checkSameOrigin