Set-up the _decoder attribute if necessary.

def _init_decoder(self):
        """
        Set-up the _decoder attribute if necessary.
        """
        # Note: content-encoding value should be case-insensitive, per RFC 7230
        # Section 3.2
        content_encoding = self.headers.get('content-encoding', '').lower()
        if self._decoder is None:
            if content_encoding in self.CONTENT_DECODERS:
                self._decoder = _get_decoder(content_encoding)
            elif ',' in content_encoding:
                encodings = [e.strip() for e in content_encoding.split(',') if e.strip() in self.CONTENT_DECODERS]
                if len(encodings):
                    self._decoder = _get_decoder(content_encoding)