Caddy frontend prefer gzip

Implements rewriting Accept-Encoding header to gzip if gzip is anyhow present (alone Accept-Encoding: gzip or mixed Accept-Encoding: deflate, gzip).

Thanks to using Caddy's http.rewrite + http.proxy's without switch no plugin was required.

Also template was amended, as its complexity increased.

Test test_prefer_gzip_encoding_to_backend for caddy-frontendhas been enabled to prove that it works.

Test 20180713-BE73C02 PASSED.