demux: adaptive: request optional compression

Allow server to compress manifests and text subtitles

modules/demux/adaptive/http/HTTPConnection.cpp

@@ -130,6 +130,7 @@ class adaptive::http::LibVLCHTTPSource : public adaptive::AbstractSource
         int formatRequest(const struct vlc_http_resource *,
                           struct vlc_http_msg *req)
         {
+            vlc_http_msg_add_header(req, "Accept-Encoding", "deflate, gzip");
             vlc_http_msg_add_header(req, "Cache-Control", "no-cache");
             if(range.isValid())
             {