fetcher: Send Accept-Encoding: gzip when downloading summary

fetcher: Send Accept-Encoding: gzip when downloading summary

The summary file can get large, but it compresses well (something
which is not true of other files in the ostree repo which are
already compressed). By sending Accept-Encoding: gzip (and
handling the compressed results) we send a lot less data.

I set up the flathub repo (http://flathub.org/repo) to enable
gzip for the summary file (only), and the result is that the
331514 byte large summary was transferred in 122889 bytes.
On my (fast) network this decreased the time i took to do
"flatpak remote-ls flathub" by about 100msec.

This fixes https://github.com/ostreedev/ostree/issues/802

Closes: #882
Approved by: cgwalters