fix for dump headers

[Tomi Ollila]

On Tue, Mar 28 2017, David Bremner <david at tethera.net> wrote:

> When I added message properties to the dump output I messed up the
> copy-paste to add properties to the header. The fix is trivial, but
> I'm not sure if this means we should bump the format version
> (i.e. write #notmuch-dump:3) to denote the header can be relied
> upon. I guess it doesn't hurt to do so. Opinions?

I'd say bump the format version... so rest of the series LGTM.

Tomi