Fix isPlainObject.

Author: typhonrt

src/functions.ts

@@ -401,7 +401,10 @@ export function isObject(value: unknown): value is Record<string, unknown>
  */
 export function isPlainObject(value: unknown): value is Record<string, unknown>
 {
-   return Object.prototype.toString.call(value) === '[object Object]';
+   if (Object.prototype.toString.call(value) !== '[object Object]') { return false; }
+
+   const prototype: any = Object.getPrototypeOf(value);
+   return prototype === null || prototype === Object.prototype;
 }
 
 /**

test/src/functions.test.ts

@@ -632,10 +632,13 @@ describe('ObjectUtil:', () =>
 
    it('isPlainObject', () =>
    {
+      class Test {}
+
       assert.isFalse(ObjectUtil.isPlainObject(false));
       assert.isFalse(ObjectUtil.isPlainObject(null));
       assert.isFalse(ObjectUtil.isPlainObject(void 0));
       assert.isFalse(ObjectUtil.isPlainObject(new String('test')));
+      assert.isFalse(ObjectUtil.isPlainObject(new Test()));
 
       assert.isTrue(ObjectUtil.isPlainObject({}));
       assert.isTrue(ObjectUtil.isPlainObject(Object.create(null)));