Breaks XML-RPC when Jetpack is active

If Jetpack is active on the site, your plugin causes a problem with the xmlrpc.php file in WordPress core.

You can see the problem by:

1. Installing and activating Jetpack
2. Visit https://yoursite.com/xmlrpc.php
3. You will see something like:

XML-RPC server accepts POST requests only.	<div style="display:none">
	</div>
<link rel='stylesheet' id='jetpack_css-css'  href='https://site.com.com/wp-content/plugins/jetpack/css/jetpack.css?ver=4.4.2' type='text/css' media='all' />

When working properly, xmlrpc.php should show just this when visiting it in the browser:

XML-RPC server accepts POST requests only.

This problem can be fixed by updating the onShutdown() method in src/SupsysticTables/Tables/Module.php to:

public function onShutdown() {
    if ( ! (defined('DOING_AJAX') && DOING_AJAX) && !isset($_GET['export-supsystic-table']) && !did_action('wp_footer') && ! defined( 'XMLRPC_REQUEST' ) && ! XMLRPC_REQUEST ) {
        wp_footer();
    }
}

Just need to add a check for the XMLRPC_REQUEST constant.