[FIX] event: prefer registration email over partner email

As of rev. ca989b6548, the default
recipients of a model record are used to filter out blacklisted and
duplicate emails during a mass-mailing. This can match both either
partner_ids or emails, as returned by
`message_get_default_recipients()`.

The problem with even registration is that a single partner, sometime
anonymous (Public User) can be used to register several participants.
We therefore need to override `message_get_default_recipients()` to that
intent, similarly to what is done in crm.lead since
ca989b6548.

Without this, mass-mailings sent to free event attendees would never be
sent, as they would all appear to be duplicates of each other.

addons/event/models/event.py

@@ -401,6 +401,17 @@ class EventRegistration(models.Model):
             pass
         return recipients
 
+    @api.multi
+    def message_get_default_recipients(self):
+        # Prioritize registration email over partner_id, which may be shared when a single
+        # partner booked multiple seats
+        return {
+            r.id: {'partner_ids': [],
+                   'email_to': r.email,
+                   'email_cc': False}
+            for r in self
+        }
+
     @api.multi
     def action_send_badge_email(self):
         """ Open a window to compose an email, with the template - 'event_badge'