HTML 'class' attribute should be an array in Drupal 7 (can lead to fatal errors)

In Drupal 7, the 'class' attribute needs to be an array, not a string. The attached patch attempts to convert all instances in Webform that weren't converted yet.

I'm marking this major because in certain situations, this can cause fatal errors (e.g., if $class = 'some-class' and then another part of Drupal runs $class[] = 'another-class'). I have seen this occur on the webform results page sometimes, although I'm not sure exactly what the trigger is.