RArray<T> should implement r.Filter<T>

[marshall007]

It looks like you aren't able to call .filter(...) on an SequenceLike<T> since that type includes RArray<T>, which does not implement r.Filter<T>.

@blakeembrey I can't think of a reason why filter would not be available on RArray<T>. I can PR a fix really quick, but wanted to run it by you first in case I'm missing something.

[blakeembrey]

Go for it 😄 I'm not actively using RethinkDB now so if you'd like to take on the definition, feel free to do so also!