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Use `.length` instead of `lodash.size`

`lodash.size` is better used for collections. By using `.length`, it's more
obvious that `platform` is a string.
Don't modify the response data

API-breaking change here. As of this commit, node-resteemo no longer modifies
RESTeemo's response. I decided to do this because contructing a pretty Object
from huge responses like `recent_games` would take an excessive amount of code,
and in the end I'm sure people would rather have the raw response. That's what
they expect from an API wrapper.