CRM_Core_Payment_BaseIPN::completeTransaction() should clear the Smarty template

There are occasions when CRM_Core_Payment_BaseIPN::completeTransaction() is called multiple times in the same request (in my case it is processing recurring payments using the Ewayrecurring payment processor).

When the processor runs for mulitple payments the receipts are written using a Smarty template, but the Smarty TPL variables aren't cleared before each receipt, so details from one receipt can spill over into the next.

The following code needs to be inserted before the call to self::SendMail() ...

    $template = CRM_Core_Smarty::singleton();
    $template->clearTemplateVars();

Note: the deprecated function CRM_Core_Payment_BaseIPN::updateContributionStatus() did this, so I assume it's appropriate here.

I can supply a PR, but perhaps someone can advise if this is the right approach?