Ensure gssapi_session actually contains MagBearerToken

Fallout from #98

Signed-off-by: Robbie Harwood <rharwood@redhat.com>
Reviewed-by: Simo Sorce <simo@redhat.com>
Closes #100

Author: frozencemetery

tests/t_spnego.py

@@ -12,3 +12,7 @@
     r = sess.get(url, auth=HTTPKerberosAuth())
     if r.status_code != 200:
         raise ValueError('Spnego failed')
+
+    c = r.cookies
+    if not c.get("gssapi_session").startswith("MagBearerToken="):
+        raise ValueError('gssapi_session not set')