fix truncation in ActiveRecord::Calculations

This PR fixes a wrong conversion to column_type in
`ActiveRecord::Calculations#calculation`
when the calculation has complex fields and the result
is a different type than the column.

This prevent truncation of result and allows the user
to have the original data type of result

Fixes #12937.

  Before:

      Account.sum("2.1 * accounts.credit_limit") => 667

  After:

      Account.sum("2.1 * accounts.credit_limit") => 667.8