Do not type cast twice on attribute assignment

The definition of `write_attribute` in dirty checking ultimately leads
to the columns calling `type_cast` on the value to perform the
comparison. However, this is a potentially expensive computation that we
cache when it occurs in `read_attribute`. The only case that we need the
non-type-cast form is for numeric, so we pass that through as well
(something I'm looking to remove in the future).

This also reduces the number of places that manually access various
stages in an attribute's type casting lifecycle, which will aid in one
of the larger refactorings that I'm working on.
I see. So we need to update the comment above it. Now it says: `Types which cannot typecast arbitrary values should override this method.`