Fixed #18861 -- Triggered message validation with locmem email backend

Thanks Bruno Renié for the report and the initial patch.

django/core/mail/backends/locmem.py

@@ -20,5 +20,7 @@ class EmailBackend(BaseEmailBackend):
 
     def send_messages(self, messages):
         """Redirect messages to the dummy outbox"""
+        for message in messages:  # .message() triggers header validation
+            message.message()
         mail.outbox.extend(messages)
         return len(messages)