Resolve required parameter after optional in Message

Resolve required parameter after optional in Message

It looks like this parameter has had a default since it was first
created, and somehow kept it even after a new parameter was added
to the end. codesearch.wmflabs.org suggests, and I could not find,
and calls to this function from any other code that would need
the defaulted param (probably because they fail due to the later
required parameter).

Change-Id: Iae6e736c5c957d9fccb6ab5813a3b5b02eeb3f6f