add --proxy-host-header option

contrib/google_auth_proxy.cfg.example

@@ -14,6 +14,9 @@
 
 ## pass HTTP Basic Auth, X-Forwarded-User and X-Forwarded-Email information to upstream
 # pass_basic_auth = true
+## pass the request Host Header to upstream
+## when disabled the upstream Host is used as the Host Header
+# pass_host_header = true 
 
 ## Google Apps Domains to allow authentication for
 # google_apps_domains = [