code: @Nullable
    public Decompressor lookupDecompressor(String messageEncoding) {
      DecompressorInfo info = decompressors.get(messageEncoding);
      return info != null ? info.decompressor : null;
    }
docs: Returns a decompressor for the given message encoding, or {@code null} if none has been
  registered.
  
  <p>This ignores whether the compressor is advertised.  According to the spec, if we know how
  to process this encoding, we attempt to, regardless of whether or not it is part of the
  encodings sent to the remote host.