ProtocolBinding in AuthnRequest indicates how response should be delivered, not how the request is being delivered.

Saml2Client.prepare_for_authenticate() needs to distinguish between
the request binding (how the request will be delivered) and the
response binding (how the response should be delivered,
indicated by ProtocolBinding in the request XML).
Merge pull request #31 from rectalogic/response_binding

Support different request and response bindings for AuthnRequest