code: function classUpdateChecked($tag, $el, options) {
          // setTimeout() introduced by #357
          setTimeout(function () {
              var c = "checked",
                  isChecked = $el.is(":" + c);
  
              if(!$el.attr("readonly")) {
                  if ($el.prop) {
                      // jQuery 1.6+
                      $el.prop(c, isChecked);
                  } else {
                      // jQuery 1.5 and below
                      if (isChecked) {
                          $el.attr(c, c);
                      } else {
                          $el.removeAttr(c);
                      }
                  }
              }
              
              classUpdate($tag, options.checkedClass, isChecked);
          }, 1);
      }
docs: Updating the "checked" property can be a little tricky.  This
  changed in jQuery 1.6 and now we can pass booleans to .prop().
  Prior to that, one either adds an attribute ("checked=checked") or
  removes the attribute.
  
  @param {jQuery} $tag Our Uniform span/div
  @param {jQuery} $el Original form element
  @param {Object} options Uniform options for this element