Quietly ignore extra personalisation

> If a user makes an API request with additional personalisation fields,
> we should simply discard any fields that the template doesn't have.
>
> This gives a couple of related advantages:
>
> - modifying template parameters no longer requires downtime for
>   clients - as they can pass in extra new parameters before a template
>   change, or continue passing in old unused parameters after removing
>   them from a template
>
> - services can pass in large user objects, for example, and then play
>   around with templates adding and removing fields at will
>
> we should make sure we still return an error if a user doesn't pass in
> a required parameter.

– https://www.pivotaltracker.com/story/show/140774195

tests/app/template/test_rest.py

@@ -374,8 +374,9 @@ def test_should_get_a_single_template(
             'about your ((thing))',
             'hello ((name)) we’ve received your ((thing))',
             '/service/{}/template/{}/preview?name=Amala&thing=document&foo=bar',
-            None, None,
-            'Personalisation not needed for template: foo'
+            'about your document',
+            'hello Amala we’ve received your document',
+            None,
         )
     ]
 )