Update date boost not applied

For both 7.x-1.x and 6.x-3.x, the changed boost uses the wrong field.

  // Boost for a more recent comment or node edit.
  list($changed_steepness, $changed_boost) = explode(':', $changed_settings);
  if ($changed_boost) {
    $ab = 4 / $changed_steepness;
    $params['bf'][] = "recip(ms(NOW,ds_created),3.16e-11,$ab,$ab)^$changed_boost";
  }

So the changed date boost gets applied to the creation date field.