Update Subscription URL not included in online receipt for authorize.net

In what appears to be a regression from CRM-18027, the link to update a subscription is not
being included in an online receipt.

The receipt code includes this if statement:

{if $is_recur and ($contributeMode eq 'notify' or $contributeMode eq 'directIPN')}

It only includes the update subscription link if this condition is met.

Prior to the change made in CRM-18027, contributeMode was always set to notify so it always was triggered.

However, CRM-18027 added the following to CRM/Contribute/BAO/Contribution.php:

+    // For some unit tests contribution cannot contain paymentProcessor information
+    $billingMode = empty($this->_relatedObjects['paymentProcessor']) ? CRM_Core_Payment::BILLING_MODE_NOTIFY : $this->_relatedObjects['paymentProcessor']['billing_mode'];
+    $template->assign('contributeMode', CRM_Utils_Array::value($billingMode, CRM_Core_SelectValues::contributeMode()));

Now, contributeMode is set to whatever billing_mode is set to by the payment processor. In the case of authorize.net, it is set to: 1

In Core_Payment we have

Core_Payment::BILLING_FORM = 1

Which according to the comments indicates that we collect the credit card on the civi page, which seems correct.

And Core_SelectValues::contributeMode assigns the key BILLING_MODE_FORM to "direct".

So... the template now gets "direct" instead of "notify" and therefore never displays the block.