Don't call #to_h on session in callback

**Why**: The rack session object does not respond to that method in some
versions

Author: jmhooper

lib/omniauth/login_dot_gov/callback.rb

@@ -4,7 +4,7 @@ class Callback
       attr_reader :session, :client, :id_token, :userinfo
 
       def initialize(session:, client:)
-        @session = session.to_h
+        @session = session
         @client = client
       end
 
@@ -15,7 +15,7 @@ def call(params)
           code: params['code'],
           client: client
         ).request_id_token
-        id_token.verify_nonce(session.dig('oidc', 'nonce_digest'))
+        id_token.verify_nonce(get_oidc_value_from_session('nonce_digest'))
         @userinfo = UserinfoRequest.new(
           id_token: id_token,
           client: client
@@ -29,7 +29,7 @@ def verify_state(params)
         cb_state_digest = OpenSSL::Digest::SHA256.base64digest(cb_state)
         return if SecureCompare.compare(
           cb_state_digest,
-          session.dig('oidc', 'state_digest')
+          get_oidc_value_from_session('state_digest')
         )
         raise CallbackStateMismatchError
       end
@@ -45,6 +45,12 @@ def handle_error(params)
                      end
         raise error_type, params['error_description']
       end
+
+      def get_oidc_value_from_session(key)
+        oidc_session = session['oidc']
+        return if oidc_session.nil?
+        oidc_session[key]
+      end
     end
   end
 end