Validation for single value in 'class' option

Thanks for reporting this. This is not a bug of this module. Class must be an array as specified in the core documentation of theme_link function: https://api.drupal.org/api/drupal/includes%21theme.inc/function/theme_link/7.x

class: must be declared in an array. Example: 'class' => array('class_name1','class_name2').

We are not going to "babysit" other modules/themes which does not follow documentation. The module/theme which caused this must be fixed.