[WIP] Workaround for bug in QT when proxied request accepts "gzip" 

I saw that when using splash as HTTP proxy for rendering HTML.

What I found out experimentally:

- if you use "Accept-Encoding: gzip", fails
- if you use "Accept-Encoding: deflate", works
- if you use "Accept-Encoding: deflate,gzip", fails
- if you do NOT set "Accept-Encoding", works

Reference:

- http://qt-project.org/forums/viewthread/2357
- https://bugs.webkit.org/show_bug.cgi?id=58727