def _convert_field(self, key, value):
        """Convert value using self.converters[key].

        If `key` is not present in self.converters, a standard unicode decoding
        will be attempted.  If the conversion (either key-specific or the
        default one) fails with a ValueError, the original bytes object will be
        returned.
        """
        convert = self.converters.get(key, bytes.decode)
        try:
            return convert(value)
        except ValueError:
            # Leave in default bytes
            return value
Convert value using self.converters[key].

        If `key` is not present in self.converters, a standard unicode decoding
        will be attempted.  If the conversion (either key-specific or the
        default one) fails with a ValueError, the original bytes object will be
        returned.