Support AttributeConsumingServiceIndex in AuthnRequest

Duplicate #95 but with rebased code

>As far as I can tell it currently isn't possible to set AttributeConsumingServiceIndex=some_value for an AuthnRequest, as I only see a mention of AttributeConsumingServiceIndex in the saml20protocol_schema.xsd.

>I guess this could be implemented in a manner similar to IsPassive in 8a825f3 or https://github.com/onelogin/ruby-saml/issues/92 ?