Uniqueness validation #14
Description
Today, I noticed that when using validates :field, uniqueness: true on a model with belongs_to_multitenant doesn't cause the validation query to be restricted to the tenant.
My goal was a tenant-scoped uniqueness, but one could argue that application wide uniqueness can be meaningful in multitenant applications as well. In that spirit, I don't think it is a bug, but a sentence like "Multitenant doesn't affect Rails built-in uniqueness validation; for tenant-scoped validation use validates :field, uniqueness: { scope: :tenant_id }" in the readme would be helpful.