Fix notice if strict mode enabled

If gravity forms is enabled, and WP_DEBUG is true whilst running WordPress 3.6, you get notices and WP Remote is unable to connect due to notices in the response:

    <b>Strict Standards</b>:  Only variables should be passed by reference in <b>/srv/www/loggly.dev/wp-content/plugins/wpremote/wprp.plugins.php</b> on line <b>206</b><br />