Drop UPGRADE State, close #1347

Motivation:

The UPGRADE State is only returned by `WebSocketUpgradeHandler::
onStatusReceived` which is internal and that can only return 2 values,
so it could return CONTINUE instead.

Modifications:

* Drop UPGRADE
* Use CONTINUE instead

Result:

More simple code

Author: slandelle

client/src/main/java/org/asynchttpclient/AsyncHandler.java

@@ -58,11 +58,7 @@ enum State {
         /**
          * Continue the processing
          */
-        CONTINUE,
-        /**
-         * Upgrade the protocol.
-         */
-        UPGRADE
+        CONTINUE
     }
 
     /**

client/src/main/java/org/asynchttpclient/netty/handler/WebSocketHandler.java

@@ -90,7 +90,7 @@ public void call() throws Exception {
             if (connection == null)
                 connection = response.headers().get(CONNECTION);
             boolean validConnection = HttpHeaderValues.UPGRADE.contentEqualsIgnoreCase(connection);
-            boolean statusReceived = handler.onStatusReceived(status) == State.UPGRADE;
+            boolean statusReceived = handler.onStatusReceived(status) == State.CONTINUE;
 
             if (!statusReceived) {
                 try {

client/src/main/java/org/asynchttpclient/ws/WebSocketUpgradeHandler.java

@@ -67,7 +67,7 @@ public final State onBodyPartReceived(HttpResponseBodyPart bodyPart) throws Exce
     @Override
     public final State onStatusReceived(HttpResponseStatus responseStatus) throws Exception {
         status = responseStatus.getStatusCode();
-        return status == SWITCHING_PROTOCOLS ? State.UPGRADE : State.ABORT;
+        return status == SWITCHING_PROTOCOLS ? State.CONTINUE : State.ABORT;
     }
 
     @Override