$options argument for xmlrpc() changed to a headers array

Due to the replacement of drupal_http_request() with Guzzle, the $options argument for xmlrpc() that was directly passed to drupal_http_request() has been changed to an array of headers instead.

7.x

xmlrpc($url, $args, array('header' => array('X-My-Awesome-Header' => 'Awesomeness')));

8.x

xmlrpc($url, $args, array('X-My-Awesome-Header' => 'Awesomeness'));