combining gzipped files

[zuozp8]

I have few processes uploading parts of a big json-lines file and I upload them compressed, as described in transparent encoding. After those partial uploads I'm combining the file into the final big compressed file, and I would like to set the metadata.contentEncoding during the /compose request.

Concatenated gzip files creates valid gzip file; I verified that if I add content encoding manually to combined file then the decompressive transcoding works as intended.

I'll provide pull-request