src/network: let curl handle the HTTP Content-Encoding
src/network: let curl handle the HTTP Content-Encoding

I would like to consume RAUC bundles straight from our CI environment
which uses OpenStack Swift as a backend for artifact storage, and
OpenDev's Zuul as the CI runner. As it happens, our CI jobs sends file
content to Swift with a Content-Encoding: gzip [1], and Swift appears to
always send the file with that Content-Encoding no matter what
Accept-Encoding the HTTP client sends in their request.

This patch simply uses curl's list of default supported Content-Encoding
decoders (that's the special magic "" value of this header). The end
result is that our boxes can now fetch stuff from our CI environment
without any problems.

TL;DR: this should save some bandwidth at essentially a zero cost, and
it unbreaks updates from network under at least one scenario.

[1] https://review.opendev.org/688154

Signed-off-by: Jan Kundrát <3d8a9088e860442addf8ae206a9abb8b0f277291@cesnet.cz>

rauc/rauc,ejoerns/rauc,ejoerns/rauc,rauc/rauc,ejoerns/rauc,rauc/rauc,jluebbe/rauc,jluebbe/rauc,jluebbe/rauc