Respect the AsyncRequestProducer contract by properly implementing available() method (#3235)

* Respect the AsyncRequestProducer contract by properly implementing available() method

* Address code review comments

Author: reta

rt/transports/http-hc5/src/main/java/org/apache/cxf/transport/http/asyncclient/hc5/CXFHttpAsyncRequestProducer.java

@@ -94,7 +94,7 @@ private void resetRequest() {
 
     @Override
     public int available() {
-        return 0;
+        return Integer.MAX_VALUE;
     }
 
     @Override