public function testValidateDatetime()
 {
     $this->assertEquals('2015-08-07 12:13:14', RestValidatorHelper::validate_datetime(['date' => '2015-08-07 12:13:14'], 'date'));
     $this->assertEquals(null, RestValidatorHelper::validate_datetime([], 'date', ['required' => false]));
     TestHelper::assertException(function () {
         RestValidatorHelper::validate_datetime([], 'date');
     }, 'ValidationException');
     TestHelper::assertException(function () {
         RestValidatorHelper::validate_datetime(['date' => 'error'], 'date');
     }, 'ValidationException');
 }