Uniqueness validation uses a proc to specify the `:conditions` option.

This is a follow up to #5321 and follows the general direction in
AR to make things lazy evaluated.
Merge pull request #9633 from senny/5321_make_it_lazy

Uniqueness validation uses a proc to specify the `:conditions` option.

This is a follow up to #5321 and follows the general direction in
AR to make things lazy evaluated.