def _marshall_fields(self):
        """Convert the field dict into the string segment of field key/value
        pairs.

        :rtype: str

        """
        values = {}
        for key, value in self.fields.items():
            if (isinstance(value, int) or
                    (isinstance(value, str) and value.isdigit() and
                     '.' not in value)):
                values[key] = '{}i'.format(value)
            elif isinstance(value, bool):
                values[key] = self._escape(value)
            elif isinstance(value, float):
                values[key] = '{}'.format(value)
            elif isinstance(value, str):
                values[key] = '"{}"'.format(self._escape(value))
        return ','.join(['{}={}'.format(self._escape(k), v)
                         for k, v in values.items()])
Convert the field dict into the string segment of field key/value
        pairs.

        :rtype: str