#1436 Wrong compressed size for GZip archive

[quanta]

When listing the contents of a Gzip archive, 7-Zip incorrectly reports the file's compressed size as equal to the size of the archive. The compressed size should be smaller because an GZip archive includes header information. However, GZip archive does not store compressed size into the header, so the value needs to be deduced by subtracting the size of the archive with the size of the header. Since a GZip archive supports storing only 1 file, this should be trivial to implement.