make it more clear why we are closing connections

I/Lukasa noticed this while investigating #295. Basically the ``getattr(conn, 'sock', False)`` followed by ``if not sock`` is catching the case where the Connection: close header is set because calling ``response.close()``  sets ``sock=None``. This pull request makes it clearer when we are passing on closing due to AppEngine craziness as opposed to do to someone already calling ``response.close`` such as ``httplib`` doing it for us.