Fix trailing slash

Getting the following when verifying JWT in IDhub (possibly first service doing this?)

INFO:apigatewayauth.authentication:Incoming API token failed verification: Token has wrong audience https://webapp-{foobar}-nw.a.run.app, expected one of ['https://webapp-{foobar}-nw.a.run.app/']

Could override expected audience via env var but better the default behaves the same way as the API Gateway creates the JWT.

Part of https://gitlab.developers.cam.ac.uk/uis/devops/iam/idhub/api/-/issues/90

apigatewayauth/tests/test_apigateway_auth.py

@@ -251,6 +251,23 @@ class APIGatewayAuthTestCase(TestCase):
             self.mock_verify_token.call_args[1]["certs_url"], "https://issuer.invalid/certs"
         )
 
+    def test_default_audience(self):
+        _, auth = self.auth.authenticate(
+            self.request_with_headers(
+                {
+                    "x-api-org-name": "test",
+                    "x-api-developer-app-class": "public",
+                    "x-api-oauth2-user": str(Identifier("a123", IdentifierSchemes.CRSID)),
+                }
+            )
+        )
+        self.mock_verify_token.assert_called_once()
+        self.assertEqual(self.mock_verify_token.call_args[0][0], self.expected_token)
+        self.assertEqual(
+            set(self.mock_verify_token.call_args[1]["audience"]),
+            {"http://testserver/", "http://testserver"},
+        )
+
     def test_bad_id_token(self):
         with self.assertRaisesMessage(AuthenticationFailed, "Invalid API Gateway token"):
             self.auth.authenticate(