Fix update

[Geesu]

According to the documentation you shouldn't send the entire object, only the fields you care about:

Fixes #23

[alydersen]

Hi guys,
I would really appreciate if you could merge this branch - the error is preventing me from using the whole update function.

[Geesu]

You can just reference my branch in your gemfile if you'd like, or fork it.

[soccernee]

As mentioned in issue #23, this is a good step in the right direction, but it does constitute a breaking change. The update method currently supports making updates to the entity itself, and then passing it to the update method.

This change would only allow updates to be made by passing in an attributes hash of the updated values. What we could do, though, is if an attributes hash is supplied to the update method, only pass that up to the PW API. When such a hash does not exist, the update method should fall back to passing up the entity.

What do you think of that, @Geesu?

[Geesu]

Yea that makes complete sense. I'll update this PR shortly.