deleteChanged = function deleteChanged(value, attr) {
        if (!self._pending[attr] && !self._silent[attr]) {
          delete self.changed[attr];
        }
      }
Determine if the model has changed since the last <code>"change"</code>
event.  If you specify an attribute name, determine if that attribute
has changed.
@param {String} attr Optional attribute name
@return {Boolean}