Cleans up some deprecation notices that can clutter up tests on Ruby 2.4.0, can be placed in `config/initializers`.

ruby_2_4_0_integer_fixes.rb

# Fixes gems that reference Fixnum explicitly. Replaces `Fixnum` with `0.class`.

# mail-2.6.4/lib/mail/attachments_list.rb:33
module Mail
  class AttachmentsList < Array
    def [](index_value)
      if index_value.is_a?(0.class)
        self.fetch(index_value)
      else
        self.select { |a| a.filename == index_value }.first
      end
    end
  end
end