Clear memory cache on page disposing
Clear memory cache on page disposing

In some cases prerender generates empty page on 304 server respond.
There is clearing of localStorage before disposing phantom's page
in `server._send`, but it is necessary to clear memory cache too in
order to avoid this.

mgmco/prerender,inventaire/prerender,goabout/prerender,prerender/prerender,PaigeeWorld/prerender,netlify/prerender,goabout/prerender,inventaire/prerender,icilalune/prerender,ArnaudValensi/prerender-schedule-from-sitemap,JuoCode/prerender