Currently, `count` and `average` always returns numeric value, but
`sum`, `maximum`, and `minimum` not always return numeric value if
aggregated on custom attribute type.

I think that inconsistent behavior is surprising:

```ruby
# All adapters except postgresql adapter are affected
# by custom type casting.

Book.group(:status).sum(:status)
# => { "proposed" => "proposed", "published" => nil }
```

That is caused by fallback looking up cast type to `type_for(column)`.
Now all supported adapters can return numeric value without that
fallback, so I think we can remove that, it will also fix aggregate
functions to return numeric value consistently.