Returns a list of meta keys to be (maybe) populated in wp_update_user().

The list of keys returned via this function are dependent on the presence
of those keys in the user meta data to be set.

function _get_additional_user_keys( $user ) {
	$keys = array( 'first_name', 'last_name', 'nickname', 'description', 'rich_editing', 'syntax_highlighting', 'comment_shortcuts', 'admin_color', 'use_ssl', 'show_admin_bar_front', 'locale' );
	return array_merge( $keys, array_keys( wp_get_user_contact_methods( $user ) ) );
}