Bugfix 13853 selected items not checked

In reference to this bug: https://dev.plone.org/ticket/13853 

**from the bug:**
in debugging it... I think the error is in the comparison q(this).attr('checked') === true

where the attribute returns a string and is compared with true. see  http://stackoverflow.com/questions/10231292/jquery-no-longer-accepting-attrchecked-true-logic

  function check_referenced_items(fieldname) {
  425	      var refs_in_overlay = jq('input.insertreference'),
  426	          uid_selector = "input[name='" + fieldname + ":list']",
  427	          current = jq(uid_selector), // the widget in the form
  428	          current_uids = current.map(function () {
  429	              if (jq(this).attr('checked') === true) {
  430	                  return jq(this).attr('value');
  431	              }
  432	              return null;
  433	          });