Experiments: fix sending out multiple experiment cookies

Only classify people into an experiment when we are rewriting html.
Fixes https://github.com/pagespeed/ngx_pagespeed/issues/586

test/nginx_system_test.sh

@@ -1568,6 +1568,8 @@ EXP_EXAMPLE="http://experiment.example.com/mod_pagespeed_example"
 EXP_EXTEND_CACHE="$EXP_EXAMPLE/extend_cache.html"
 OUT=$(http_proxy=$SECONDARY_HOSTNAME $WGET_DUMP $EXP_EXTEND_CACHE)
 check_from "$OUT" fgrep "PageSpeedExperiment="
+MATCHES=$(echo "$OUT" | grep -c "PageSpeedExperiment=")
+check [ $MATCHES -eq 1 ]
 
 start_test PageSpeedFilters query param should disable experiments.
 URL="$EXP_EXTEND_CACHE?PageSpeed=on&PageSpeedFilters=rewrite_css"

test/pagespeed_test.conf.template

@@ -141,6 +141,7 @@ http {
     listen @@SECONDARY_PORT@@;
     server_name experiment.example.com;
     pagespeed FileCachePath "@@FILE_CACHE@@";
+    pagespeed InPlaceResourceOptimization off;
 
     pagespeed RunExperiment on;
     pagespeed AnalyticsID "123-45-6734";