Fix numericality validator when defined on abstract class:

[Edouard-chin]

Fix numericality validator when defined on abstract class:

• Problem

  It's no longer possible to define a numeric validation on abstract
  class:

    class AnimalsBase < ApplicationRecord
      self.abstract_class = true
  
      validates :age, numericality: { min: 18 }
    end
  
    class Dog < AnimalsBase
    end
  
    Dog.create!(age: 0) => ActiveRecord::TableNotSpecified: AnimalsBase has no table configured. Set one with AnimalsBase.table_name=

  Solution

  Instead of trying to get the type for attribute on the class
  defining the validation, get it from the record being validated.

cc/ @rafaelfranca @casperisfine @gmcgibbon