More specs for Enumerable#chunk

While going through the documentation of Enumerable#chunk I noticed that it's rubyspecs don't cover everything mentioned there.
More specifically:
- There are no specs for the keys nil, :_separator and :_alone.
- There is no spec for any other symbol that starts with an underscore.
- There is no spec for what happens if the optional argument initial_state gets passed.

So here's my pull request where I added specs for all of the situations mentioned above.