Add missing is_valid call

[Destiner]

Hi there. When I was working on adding tests, I stumbled upon update_fulfillment method. When I tried to call it from test code, it failed.

I think the reason of failing is that it should call is_valid before saving the object to DB, similar to the fulfill_bounty method just above.

Here's the method: https://github.com/Bounties-Network/BountiesAPI/blob/master/bounties_api/std_bounties/bounty_client.py#L122-L124