Fix exception when running dedupe management command

[madchap]

Fixes #4426

[valentijnscholten]

What I had as a bug a while ago is that I forgot to pass in the fields parameter somewhere, causing no fields to be updated. So I made it mandatory, but forgot dedupe.py it seems. I prefer to make the parameter mandatory, as having no fields is a non-default case and it would be good to force people to pass in fields=None if they don't want any fields to be updated.
i.e. change dedupe.py instead of utils.py :-)

[valentijnscholten]

see above

[madchap]

That's what I thought first, but then I got lazy.

[valentijnscholten]

I can fix it as it was my bug

[madchap]

It's ok, you fix enough of mine :)