createLoginRequest serializes undefined ACSUrls

[logancarmody]

For a given protocol, if the ACSUrl is undefined, createLoginRequest serializes this undefined ACSUrl. Some IdPs like Google can't handle the serialized undefined.

I discovered this because I was misconstructing the Service Provider, so this was based out of user error. However, I don't think this is the correct behavior. I think the lib should either:

• Throw an exception
• Don't serialize the ACSUrl at all since it's optional in Saml2.0

[tngan]

@logancarmody Thanks for your advice, I agree that the construction process is not that straightforward right now, we will simplify and enhance with more restricted validation when we restructure the v3 API.