AJAX callback cannot take object method due to function_exists() check

Similar issue has been fixed in Drupal 8 core, however the bug still exists in 7.

Callback works successfully when function_exists() is swapped out for is_callable().

Example:

$form['checkbox'] = array(
  '#type' => 'checkboxes',
  '#options' => $options,
  '#ajax' => array(
    'callback' => array($this, 'ajaxCallbackFunction'),
  ),
);