Fix #2990: Wiki Autocomplete Misses On Certain Meta-wikis

/wiki_pages?search[order]=post_count didn't include wiki pages that
didn't belong to a tag. This was due to doing an inner join on the tags
table instead of a left outer join.

test/functional/wiki_pages_controller_test.rb

@@ -28,6 +28,11 @@ class WikiPagesControllerTest < ActionController::TestCase
         get :index, {:search => {:title => "abc"}}
         assert_redirected_to(wiki_page_path(@wiki_page_abc))
       end
+
+      should "list wiki_pages without tags with order=post_count" do
+        get :index, {:search => {:title => "abc", :order => "post_count"}}
+        assert_redirected_to(wiki_page_path(@wiki_page_abc))
+      end
     end
 
     context "show action" do