want_assertions_encrypted should add a validation to require IdP encryption

[johnnyshields]

The want_assertions_encrypted currently only controls whether encryption-related certs are included in the SP metadata. There is no validation that the IdP is actually using encryption.

Compare with want_assertions_signed, which does add such a validation:

     # in response.rb
      if settings.security[:want_assertions_signed] && !(signed_elements.include? "Assertion")
        return append_error("The Assertion of the Response is not signed and the SP requires it")
      end