Proxy.hs: allow http-reverse-proxy 0.6.0
Proxy.hs: allow http-reverse-proxy 0.6.0

This patch allows keter to be compatible with http-reverse-proxy 0.6.0,
without losing compatibility with earlier versions of http-reverse-proxy.

snoyberg/keter,snoyberg/keter,tolysz/keter,tolysz/keter