add params to metadata action to fix multi IdPs issue

[luke-zhou]

For issue #173, support metadata for multiple IdPs

[samgreen96]

In regards to this change would it be worth going one step further and leveraging the exisiting get_idp_entity_id(params) method in place of params[:idp_entity_id] for the use case where someone has implemented their own idp_entity_reader class?

Sorry to comment on a closed issue - and thanks very much for making this gem :)

[adamstegman]

@samgreen96 it would definitely be nice! Unfortunately the default entity ID reader depends on the presence of a SAML payload—I think it would make sense to enhance it to look for the presence of an idp_entity_id query parameter if you want to adjust that.