ActiveRecord -> Active Record

As per the Rails documentation guidelines.
ARel -> Arel

The project uses "Arel" most of the time, but there were a few "ARel"
here and there. I checked with @brynary back in 2010 to pick one for
the Rails documentation guidelines and "Arel" was chosen and documented.
This patch chooses "Arel" vs "ARel" based on that.