Only use valid kwargs for update and insert
@tax, thank you for your message and the PR, I'm glad you're liking using "peewee".

The KeyError is intentional -- I think its better to raise an exception than not insert/update something because the user made a typo.

Hi Charles, thanks for creating this excellent lightweight project, it is super simple and fun to work with.

I ran in to a problem in a project when passing on a dict to the update query of a model this raised a KeyError because one of the keys in the dict was not a field in the model.
This could be by design, if not I hope you will merge this pull request.

Again thanks for all the hard work on this project.
