Dynamic length validator

Теперь в associated_length_validator можно передавать лямбду или символ, как 

  validates :field, association_length: { maximum: -> (record) { record.max } }
  validates :field2, association_length: { maximum: -> { 3 } }
  validates :field3, association_length: { maximum: :max }

UPD. пофиксил тесты для ruby1.9.3
assignment in condition is evil. it requires to think what it does every time you see it. please rewrite it.