lib/mandrill_dm/message.rb
def get_value(field)
      if mail[field].respond_to?(:unparsed_value)                     # `mail` gem > 2.7.0
        mail[field].unparsed_value
      elsif mail[field].instance_variable_defined?('@unparsed_value') # `mail` gem = 2.7.0
        mail[field].instance_variable_get('@unparsed_value')
      elsif mail[field].instance_variable_defined?('@value')          # `mail` gem < 2.7.0
        mail[field].instance_variable_get('@value')
      end
    end
def get_value(field)
      if mail[field].respond_to?(:unparsed_value)                     # `mail` gem > 2.7.0
        mail[field].unparsed_value
      elsif mail[field].instance_variable_defined?('@unparsed_value') # `mail` gem = 2.7.0
        mail[field].instance_variable_get('@unparsed_value')
      elsif mail[field].instance_variable_defined?('@value')          # `mail` gem < 2.7.0
        mail[field].instance_variable_get('@value')
      end
    end
rubocop:disable Metrics/AbcSize