cleanup singleton-instance of HttpAsyncClientSingleton.HTTP_CLIENT by setting HTTP_CLIENT to null on ConnectorCommon.shutdown()

Author: lucnygr

src/main/java/org/aarboard/nextcloud/api/utils/ConnectorCommon.java

@@ -273,6 +273,7 @@ public interface ResponseParser<R> {
     public static void shutdown() throws IOException {
             if(HttpAsyncClientSingleton.HTTPC_CLIENT != null) {
                 HttpAsyncClientSingleton.getInstance(null).close();
+                HttpAsyncClientSingleton.HTTPC_CLIENT = null;
             }		
     }
 }