XML-RPC client does not work with community hub

Though we dropped Zend Framework's XML-RPC client and server and implemented our own using PHP's native XML-RPC functionalities, we need to make sure that everything still works just the same, especially the interoperability with community hub sites.

In this case, the current master's implementation of the XML-RPC web service does not work with older sites using the Zend XML-RPC libraries.

To reproduce:

1. On the Dashboard, add the Community finder block.
2. Click the Search link in the Community finder block.
3. On the Search for community course page, click the Search button.
   • An error message comes up: An error occurred when retrieving the course listing from the selected hub, please try again later. (Unknown error)