adds HistoryArray#to_h and spec

I was looking at the output of _out_.to_a because I wanted a specific item, but it was hard to determine how to get at that particular return via _out_[]. Turns out all I had to do was expose the internal @hash being used. Simply returning the object reads nicer, but I made it deep copy because that seemed like proper form. Also added a spec for coverage; can certainly elaborate on that if necessary.