Don't include nil in the return value of Sisimai::MTA::*.scan() method

[azumakuniyuki]

Initial value of the following keys have nil

      def DELIVERYSTATUS
        return {
          'spec'         => nil,  # Protocl specification
          'date'         => nil,  # The value of Last-Attempt-Date header
          'rhost'        => nil,  # The value of Remote-MTA header
          'lhost'        => nil,  # The value of Received-From-MTA header
          'alias'        => nil,  # The value of alias entry(RHS)
          'agent'        => nil,  # MTA name
          'action'       => nil,  # The value of Action header
          'status'       => nil,  # The value of Status header
          'reason'       => nil,  # Temporary reason of bounce
          'command'      => nil,  # SMTP command in the message body
          'diagnosis'    => nil,  # The value of Diagnostic-Code header
          'recipient'    => nil,  # The value of Final-Recipient header
          'softbounce'   => nil,  # Soft bounce or not
          'feedbacktype' => nil,  # Feedback Type
        }
      end

When there is any value which is still nil or could not be detected, each value should be set ''(empty string).