fix(test): use string based id in one compliance test for coverage

Author: jankapunkt

test/compliance/refresh-token-grant-type_test.js

@@ -73,7 +73,7 @@ const auth = new OAuth2Server({
 });
 
 const user = db.saveUser({ id: 1, username: 'test', password: 'test'});
-const client = db.saveClient({ id: 1, secret: 'b', grants: ['password', 'refresh_token'] });
+const client = db.saveClient({ id: '1', secret: 'b', grants: ['password', 'refresh_token'] });
 const scope = 'read write';
 
 function createLoginRequest () {