posts: fix exception in random:1 searches.

Fix regression in 1ad0e8688. Caused by `relation.order_values` returning
an array of Arel nodes instead of an array of strings when doing a
`random:1` search.

app/logical/post_query_builder.rb

@@ -542,7 +542,9 @@ class PostQueryBuilder
   # better.
   def optimize_search(relation, small_search_threshold)
     return relation unless small_search_threshold.present?
-    return relation unless relation.order_values.map(&:downcase).in?([["posts.id desc"], ["posts.id asc"]])
+
+    order_values = relation.order_values.map { |order| order.try(:to_sql) || order.to_s }.map(&:downcase)
+    return relation unless order_values.in?([["posts.id desc"], ["posts.id asc"]])
 
     if post_count.nil?
       # If post_count is nil, then the search took too long to count and we don't

test/functional/posts_controller_test.rb

@@ -323,6 +323,9 @@ class PostsControllerTest < ActionDispatch::IntegrationTest
           get posts_path, params: { tags: "order:random" }
           assert_response :success
 
+          get posts_path(tags: "random:1")
+          assert_response :success
+
           get posts_path(random: "1")
           assert_redirected_to posts_path(tags: "random:20", format: :html)