Pass allow unknown flag to row scanner.

[popescu-af]

• Pass the flag to row scanner
• Add test
• Extra: initialize row scanner targets array with valid pointers,
  because it is not guaranteed that all drivers support scanning into nil
  (golang's database/sql does not for example).

[stephenafamo]

I would rather not add the extra initialization every time. This would add extra allocations even when they are not needed.

Instead, it would be better to only initialize if the column is skipped, i.e. if allowUnknown is true and there is no scanDestination.