Modify headers to match case used in RFC examples

Change the subprotocol and extension header names to match the case used
in RFC examples. Other headers names already match the case used in the
examples.

Although the headers names in the handshake are case insensitive, some
libraries expect the exact case used in the RFC examples. This change
allows the package to interoperate with those broken libraries.

client.go

@@ -201,7 +201,7 @@ func (d *Dialer) Dial(urlStr string, requestHeader http.Header) (*Conn, *http.Re
 	}
 
 	if d.EnableCompression {
-		req.Header.Set("Sec-Websocket-Extensions", "permessage-deflate; server_no_context_takeover; client_no_context_takeover")
+		req.Header["Sec-WebSocket-Extensions"] = []string{"permessage-deflate; server_no_context_takeover; client_no_context_takeover"}
 	}
 
 	var deadline time.Time

server.go

@@ -103,7 +103,7 @@ func (u *Upgrader) selectSubprotocol(r *http.Request, responseHeader http.Header
 //
 // The responseHeader is included in the response to the client's upgrade
 // request. Use the responseHeader to specify cookies (Set-Cookie) and the
-// application negotiated subprotocol (Sec-Websocket-Protocol).
+// application negotiated subprotocol (Sec-WebSocket-Protocol).
 //
 // If the upgrade fails, then Upgrade replies to the client with an HTTP error
 // response.
@@ -127,7 +127,7 @@ func (u *Upgrader) Upgrade(w http.ResponseWriter, r *http.Request, responseHeade
 	}
 
 	if _, ok := responseHeader["Sec-Websocket-Extensions"]; ok {
-		return u.returnError(w, r, http.StatusInternalServerError, "websocket: application specific 'Sec-Websocket-Extensions' headers are unsupported")
+		return u.returnError(w, r, http.StatusInternalServerError, "websocket: application specific 'Sec-WebSocket-Extensions' headers are unsupported")
 	}
 
 	checkOrigin := u.CheckOrigin
@@ -140,7 +140,7 @@ func (u *Upgrader) Upgrade(w http.ResponseWriter, r *http.Request, responseHeade
 
 	challengeKey := r.Header.Get("Sec-Websocket-Key")
 	if challengeKey == "" {
-		return u.returnError(w, r, http.StatusBadRequest, "websocket: not a websocket handshake: `Sec-Websocket-Key' header is missing or blank")
+		return u.returnError(w, r, http.StatusBadRequest, "websocket: not a websocket handshake: `Sec-WebSocket-Key' header is missing or blank")
 	}
 
 	subprotocol := u.selectSubprotocol(r, responseHeader)
@@ -190,12 +190,12 @@ func (u *Upgrader) Upgrade(w http.ResponseWriter, r *http.Request, responseHeade
 	p = append(p, computeAcceptKey(challengeKey)...)
 	p = append(p, "\r\n"...)
 	if c.subprotocol != "" {
-		p = append(p, "Sec-Websocket-Protocol: "...)
+		p = append(p, "Sec-WebSocket-Protocol: "...)
 		p = append(p, c.subprotocol...)
 		p = append(p, "\r\n"...)
 	}
 	if compress {
-		p = append(p, "Sec-Websocket-Extensions: permessage-deflate; server_no_context_takeover; client_no_context_takeover\r\n"...)
+		p = append(p, "Sec-WebSocket-Extensions: permessage-deflate; server_no_context_takeover; client_no_context_takeover\r\n"...)
 	}
 	for k, vs := range responseHeader {
 		if k == "Sec-Websocket-Protocol" {