Select "Enable multiple bulk email address for a contact", "hold_date" can not be updated

Steps to reproduce:

1. Open the page „Administer CiviCRM“ --> “CiviMail Component Settings”, and select the option “Enable multiple bulk email address for a contact” (1_admin_civimail_settings.png)
2. Open a contact and set “on hold” to the email (i.g. select “On Hold Opt Out”), and check the hold_date with api, ), in my test the “hold_date” is “2017-02-22 09:27:56”, and “on_hold=2 (see the screenshots 2_set_onHold_email.png and 3_check_holddate_1.png)
3. On the page of contact, reset the “On Hold” and save it, and check the value with api, and the value of “on_hold” is 0, BUT the “hold_date” IS NOT CHANGED! The “hold_date” should be deleted, and set the current time for “reset_date”! (see the screenshots 4_delete_onHold.png and 5_check_holddate_2.png)
4. As a following error: the “hold_date” is the old one, not changed, if you set “on hold” to the email again (i.g. select “On Hold Bounce”). In my test the “hold_date” is still “2017-02-22 09:27:56”, and “on_hold=1 (see the screenshots 6_check_holddate_3.png)

To fix: ../sites/all/modules/civicrm/CRM/Core/BAO/Email.php
public static function holdEmail(&$email) {……
elseif ($email->on_hold == 'null') {

SHOULD BE

elseif ($email->on_hold == 'null'* || empty($email->on_hold*)) {