Custom classes for tags don't work if entered as strings

tagClass option bug - turn a tagClass option string into a function that returns the string.
The following example in the documentation doesn't work:

$('input').tagsinput({
  tagClass: 'big'
});

Right now, you have to enter it as 
$('input').tagsinput({
  tagClass: function(x){return 'big';}
});