convert utils/isPlainObject test to typescript (#3511)

Author: cellog

test/utils/isPlainObject.spec.js renamed to test/utils/isPlainObject.spec.ts

@@ -16,7 +16,7 @@ describe('isPlainObject', () => {
     expect(isPlainObject(new Date())).toBe(false)
     expect(isPlainObject([1, 2, 3])).toBe(false)
     expect(isPlainObject(null)).toBe(false)
-    expect(isPlainObject()).toBe(false)
+    expect(isPlainObject(undefined)).toBe(false)
     expect(isPlainObject({ x: 1, y: 2 })).toBe(true)
   })
 })