Allow passing a Proc or Symbol as an argument to length validator values

[prognostikos]

This brings the Length validator in line with the Numericality validator, which currently supports Proc & Symbol arguments.

An example:

class Topic
  include ActiveModel::Validations

  attr_accessor :title, :foo

  validates :title, length: { maximum: ->(topic) { topic.max_length_for_title } }

  def max_length_for_title
    if foo
      5
    else
      10
  end
end

[rails-bot]

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[prognostikos]

I would love to get rid of my freedom patch for this feature - is there any chance this PR will be accepted? Any feedback is appreciated.

[rafaelfranca]

Could you add a CHANGELOG entry and squash your commits? I'll merge after that.

[prognostikos]

@rafaelfranca done, thanks for taking care of this.