Default encoding not being shown in system settings form.

The settings form is missing the variable as default value, so changing it requires writting everytime you save the settings.

  $form['mailhandler_default_encoding'] = array(
    '#type' => 'textfield',
    '#title' => t('Default character encoding'),
    '#default_value' => 'UTF-8',
    '#description' => t('The default character encoding to use when an incoming message does not define an encoding.')
  );

should be '#default_value' => variable_get('mailhandler_default_encoding', 'UTF-8'),