deep copy empty attributes

[AndrewAXue]

Addresses #713. Previously it was possible for a user (acting against the api) to mutate a default variable. Deepcopying solves this issue.

[toumorokoshi]

I agree that this approach will fix the issue.

One fix and I think it's good to merge: can you create utility methods rather than have parameterized creation? It's pretty easy to pass the wrong variable and create the wrong object.

I'd suggest public utility methods around empty_{attributes,events,links}.

[toumorokoshi]

thanks! I'm not sure that these attributes being protected is the right approach, but until we find a need to create such empty sets outside of the constructor, I think we're ok.

[codeboten]

Can you add a note to the sdk changelog, i'll approve and merge afterwards

[codeboten]

Thanks! 👍