changset.get on a belongsTo relationship returns a proxy

Relates to #543

@snewcomer thank you for getting that fix merged in so quickly for the `mergeDeep` issue on a `hasMany`! 💪

In testing out the v3.9.2 we realized we are still experiencing this issue in `belongsTo` relationships as well.

I have to take a break from the keyboard for a bit, but here's another breaking test to give you an idea of how it is occurring.

https://github.com/jaredgalanis/ember-changeset/commit/470a1558786a1a5df707926a5d4fd9f5b571dde9

I'm happy to take a swing at fixing it later, but given your understanding of the internals it may just be more efficient if you know where the fix is to let you handle it?

Thanks again for addressing this so quickly! 😃