Support for X-Forwarded-Proto and other custom headers

I decided to take a stab at implementing a solution to this so the attached patch adds a drupal_alter call on the purge_requests array just before passing it on for processing with curl. Below is an implementation of the drupal_alter to duplicate all the purge requests and add the x-forwarded-proto header to the second set although any headers could be added at this point.

/**
 * Implements hook_purge_post_build_alter().
 *
 * Duplicate the requests and in the second set make sure to purge the HTTPS version of the page.
 */
function example_purge_post_build_alter(array &$purge_requests) {
  // 1. Copy the existing array
  $https_purge_requests = array();
  $https_purge_requests = $purge_requests;

  // 2. Add HTTPS x-forwarded-for header to the copy.
  foreach ($https_purge_requests as $key => $request) {
    if (isset($request['headers'])) {
      $https_purge_requests[$key]['headers'][] = 'X-Forwarded-Proto: https';
    }
  }

  // 3. Merge together.
  $purge_requests = array_merge($purge_requests, $https_purge_requests);
}