fix: UpdateItemOutput's Attributes shouldn't be partial if ReturnValue is set to ALL_*

[evanandrewrose]

I'm guessing this is just a copy/paste error. I checked out delete-item.ts and update-item.ts and they both look as expected (partials only with UPDATED_{NEW, OLD}).

Relevant aws docs section:

ALL_NEW - Returns all of the attributes of the item, as they appear after the UpdateItem operation. UPDATED_NEW - Returns only the updated attributes, as they appear after the UpdateItem operation.

This lib is great btw, thanks for sharing it.

[sam-goodwin]

Ah, that does look like a copy paste error. Thanks for the quick patch.