Add Accept-Encoding header to cache key

Most webservers respond with `Vary: Accept-Encoding` when presented with
the  `Accept-Encoding` header, which causes Nginx to automatically
include the request header as part of the cache key. Unfortunately,
issue #8 shows that at least GitLab does not respond with the expected
`Vary` header causing the cache key to be too broad.

For example, if you sent a specific request to Velocita with
`Accept-Encoding` set to `br`, the Brotli-encoded response would be
cached. If you would then perform the same request _without_ the
`Accept-Encoding` header, you would erroneously receive that same
Brotli-encoded response.

Fix this by always including the value of the `Accept-Encoding` request
header in the cache key.

Author: gmta

proxy/templates/proxy.conf.erb

@@ -57,7 +57,7 @@ access_log /var/log/nginx/access.log upstreamlog;
 # Caching configuration
 proxy_cache_background_update on;
 proxy_cache_convert_head off;
-proxy_cache_key $request_method$scheme$proxy_host$request_uri;
+proxy_cache_key "$request_method $scheme $http_accept_encoding $proxy_host $request_uri";
 proxy_cache_lock on;
 proxy_cache_use_stale error timeout updating http_403 http_404 http_429 http_500 http_502 http_503 http_504;
 proxy_cache_valid 200 302 1d;