Apply Python 3 changes to grid_openid allowing test instantiation.

Make use of a need to cover the basic operation of this code for some
minimal changes that allow the server it implements to be trivial to
instantiate directly as an object. Use this ability to do so under test
and add the necessary wiring to issue a single top-level request from
the server and validate it responds with no internal issues logged.

Note that a top-level request was not previously succeedding due to
certain values being errantly dereferenced (though the code was clearly
written to have been safe under these circumstances). Fix this here.