Those types are not driver.Value, but int was necessary because sqlmock
was not properly using pure driver.Value before. Now that I fixed the
issue in previous commit, we can remove the old hack.
The input of Scan is a driver.Value. Which means the only integer type
on input is a int64. But NullInt.Scan also handles int/int8/int16/int32
while that should not be necessary.
Unfortunately, sqlmock doesn't enforce strict driver.Value in its
implementation and the testsuite (that uses NullInt) relies on this bug.
So, this patch just adds a FIXME until sqlmock internals and testsuite
are fixed (will maybe require breaking the API).
