Detect if AsyncWebSocketClient can send data to server and add AsyncWebServer::end() (me-no-dev#501)

* added AsyncWebServer::end()

* added AsyncWebServer::end()

* added bool AsyncWebSocketClient::canSend()

* Fix comparison with WS_MAX_QUEUED_MESSAGES

* Fix comparison with WS_MAX_QUEUED_MESSAGES

Author: dyarkovoy

src/AsyncWebSocket.cpp

@@ -551,7 +551,7 @@ void AsyncWebSocketClient::_queueMessage(AsyncWebSocketMessage *dataMessage){
     delete dataMessage;
     return;
   }
-  if(_messageQueue.length() > WS_MAX_QUEUED_MESSAGES){
+  if(_messageQueue.length() >= WS_MAX_QUEUED_MESSAGES){
       ets_printf("ERROR: Too many messages queued\n");
       delete dataMessage;
   } else {

src/AsyncWebSocket.h

@@ -210,6 +210,8 @@ class AsyncWebSocketClient {
     void binary(const __FlashStringHelper *data, size_t len);
     void binary(AsyncWebSocketMessageBuffer *buffer); 
 
+    bool canSend() { return _messageQueue.length() < WS_MAX_QUEUED_MESSAGES; }
+
     //system callbacks (do not call)
     void _onAck(size_t len, uint32_t time);
     void _onError(int8_t);