Reimplement Response#{value, value=} using response class

This reimplements `Response#value` and `Response#value=` using the response's answer's response class.  This was necessary to properly differentiate between e.g. strings that were meant for `string_value` or `text_value`, as it's not possible to figure that out on value type alone.

This PR also adds a default scope to Response that eager-loads the Question and Answer; see the commit messages for rationale.