Always retain `__typename` during data masking

[jerelmiller]

While working on data masking for nested fragments, I stumbled on a potential pitfall with the existing solution; the data masking algorithm only retains exactly whats defined in the selection set. This means that unless __typename is defined in the fragment, this property was removed from the returned object. This is mostly ok if you're just working with the query and one level of fragments, but as you introduce nested fragments with data masking, this made it difficult to work with. If you forget to include __typename with data masking enabled, this means the from option passed to useFragment/watchFragment was invalid, so the value returned was empty. This could be solved in user-space by adding a __typename selection in the fragment, but we don't ask our users to do that with queries, nor does using watchFragment without masking behave this way today.

This PR retains __typename when present regardless of whether its present in the original document or not.

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Latest commit: 990b6554a068c4f04adb01e115ebbb028001c1df

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