search: fix post count caching for the date: metatag.

The `date:` metatag depends on the current user's time zone, so the post count for `date:` searches
needs to be cached separately for different users, otherwise it could lead to incorrect page counts
if two users with different time zones search for the same `date:` metatag at the same time.

app/logical/post_query.rb

@@ -138,7 +138,8 @@ class PostQuery
   # True if the search depends on the current user because of permissions or privacy settings.
   def is_user_dependent_search?
     metatags.any? do |metatag|
-      metatag.name.in?(%w[upvoter upvote downvoter downvote commenter comm search flagger fav ordfav favgroup ordfavgroup]) ||
+      # XXX date: is user dependent because it depends on the current user's time zone
+      metatag.name.in?(%w[date upvoter upvote downvoter downvote commenter comm search flagger fav ordfav favgroup ordfavgroup]) ||
       metatag.name == "status" && metatag.value == "unmoderated" ||
       metatag.name == "disapproved" && !metatag.value.downcase.in?(PostDisapproval::REASONS)
     end