add_additional_fields() ignores identifier if $info is a string

The argument description for views_handler_field::add_additional_fields() states that:

The key is an identifier used to later find the field alias used.

This is only true if the passed $info is an array. When $info is a string, the field name is used instead.

Example for correct behavior:

    $this->add_additional_fields(array(
      'langcode' => array('field' => 'language'),
    ));

$this->aliases now contains a key "langcode" with the field alias.

Example for wrong behavior:

    $this->add_additional_fields(array(
      'langcode' => 'language',
    ));

$this->aliases now contains a key "language" with the field alias.