def variable(name, data)
      fail(Mailgun::ParameterError, 'Variable name must be specified') if name.to_s.empty?
      jsondata = make_json data
      set_single("v:#{name}", jsondata)
    end
def variable(name, data)
      fail(Mailgun::ParameterError, 'Variable name must be specified') if name.to_s.empty?
      jsondata = make_json data
      set_single("v:#{name}", jsondata)
    end
Attaches custom JSON data to the message. See the following doc page for more info.
 https://documentation.mailgun.com/user_manual.html#attaching-data-to-messages

 @param [String] name A name for the custom variable block.
 @param [String|Hash] data Either a string or a hash. If it is not valid JSON or
                           can not be converted to JSON, ParameterError will be raised.
 @return [void]