Reset is_stopping on successfully stopping server

Author: rastating

lib/wpxf/net/http_server.rb

@@ -92,6 +92,7 @@ def stop_http_server
     @http_server_thread&.exit
     @tcp_server.close if !@tcp_server.nil? && !@tcp_server.closed?
     emit_info 'HTTP server stopped'
+    @is_stopping = false
   end
 
   # @return [Thread] thread that the server runs on when in non-blocking mode.