The `issuer` setting was renamed to `sp_entity_id` in omniauth-saml [1],
and it's been deprecated in ruby-saml since version 1.11.0, released on
July 24, 2019 [2].
The ruby-saml code currently uses:
```
def sp_entity_id
@sp_entity_id || @issuer
end
```
So setting `issuer` to the same value as `sp_entity_id` if
`sp_entity_id` is present, as we were doing, has no effect.
On the other hand, neither omniauth-saml nor ruby-saml use the
`idp_metadata_url` and `idp_metadata` settings.
[1] https://github.com/omniauth/omniauth-saml/commit/74ed8dfb3aed
[2] https://github.com/SAML-Toolkits/ruby-saml/releases/tag/v1.11.0
75f6bebc30