add serializable test

tests/register-schema.spec.ts

@@ -29,6 +29,15 @@ describe('Schema registration', () => {
     expect(schema.dict.foo.type).toBe('string');
   });
 
+  it('should be serializable', () => {
+    const schema = Config as Schema<Config>;
+    const plainObject: Partial<Schema<Config>> = JSON.parse(
+      JSON.stringify(schema),
+    );
+    expect(plainObject.type).toBe('object');
+    expect(plainObject.dict.foo.type).toBe('string');
+  });
+
   it('should put default value', () => {
     expect(new Config({}).foo).toBe('bar');
     expect(new Config({ foo: 'baz' }).foo).toBe('baz');