Dispatch handling of this event to a receiver.

        This method will invoke ``receiver._columns_streamed`` if it exists.

def dispatch(self, receiver):
        ''' Dispatch handling of this event to a receiver.

        This method will invoke ``receiver._columns_streamed`` if it exists.

        '''
        super(ColumnsStreamedEvent, self).dispatch(receiver)
        if hasattr(receiver, '_columns_streamed'):
            receiver._columns_streamed(self)