Fix #3410: Unable to create a new wiki page.

Fix `Post.fast_count(nil)` failing when the user had the "safe mode" or
"deleted post filter" options turned on.

app/models/post.rb

@@ -1136,6 +1136,7 @@ class Post < ApplicationRecord
 
   module CountMethods
     def fast_count(tags = "", options = {})
+      tags = tags.to_s
       tags += " rating:s" if CurrentUser.safe_mode?
       tags += " -status:deleted" if CurrentUser.hide_deleted_posts? && tags !~ /(?:^|\s)(?:-)?status:.+/
       tags = Tag.normalize_query(tags)

test/unit/post_test.rb

@@ -2461,13 +2461,17 @@ class PostTest < ActiveSupport::TestCase
         context "in safe mode" do
           setup do
             CurrentUser.stubs(:safe_mode?).returns(true)
+            FactoryGirl.create(:post, "rating" => "s")
           end
 
-          should "execute a search" do
-            Post.expects(:fast_count_search).once.with("rating:s", kind_of(Hash)).returns(1)
+          should "work for a blank search" do
             assert_equal(1, Post.fast_count(""))
           end
 
+          should "work for a nil search" do
+            assert_equal(1, Post.fast_count(nil))
+          end
+
           should "not fail for a two tag search by a member" do
             post1 = FactoryGirl.create(:post, tag_string: "aaa bbb rating:s")
             post2 = FactoryGirl.create(:post, tag_string: "aaa bbb rating:e")