[Modify] Polish it

websocket-sharp/WebSocket.cs

@@ -1118,11 +1118,11 @@ namespace WebSocketSharp
         }
       }
       else {
-        var valid = _protocolsRequested
+        var isValid = _protocolsRequested
                       && subp.Length > 0
                       && _protocols.Contains (p => p == subp);
 
-        if (!valid) {
+        if (!isValid) {
           message = "The Sec-WebSocket-Protocol header is invalid.";
 
           return false;