Ensure that delegated_creds is non-null when set

Previously, in the high-level API, we would unconditionally set
wrap the return value of delegated_creds, which could result in
delegated_creds being set to the default credentials when no
delegated creds were returned (since the call would be
`Credentials(None)`).  This changes the logic so that delegated_creds
stays set as `None` unless delegated_creds are actually returned.

Partial solution for #96

gssapi/sec_contexts.py

@@ -524,7 +524,10 @@ def _acceptor_step(self, token):
         res = rsec_contexts.accept_sec_context(token, self._creds,
                                                self, self._channel_bindings)
 
-        self._delegated_creds = Credentials(res.delegated_creds)
+        if res.delegated_creds is not None:
+            self._delegated_creds = Credentials(res.delegated_creds)
+        else:
+            self._delegated_creds = None
 
         self._complete = not res.more_steps