localize-date.lua: also localize page.heading.date

This ensures a unified presentation of dates.

lua/localize-date.lua

@@ -12,6 +12,9 @@ function localize_date(date_meta)
 end
 
 function Meta (meta)
+  if meta.page and meta.page.heading and meta.page.heading.date then
+    meta.page.heading.date = localize_date(meta.page.heading.date)
+  end
   meta.quality.received = localize_date(meta.quality.received)
   meta.quality.accepted = localize_date(meta.quality.accepted)
   meta.quality.published = localize_date(meta.quality.published)