Fix pagnation counting

Mongoid count doesn't respect limit until you pass true flag.

Author: jandudulski

lib/active_admin/mongoid.rb

@@ -14,6 +14,7 @@ def setup *args, &block
       require 'active_admin/mongoid/comments'
       require 'active_admin/mongoid/resource'
       require 'active_admin/mongoid/document'
+      require 'active_admin/mongoid/helpers/collection'
     end
   end
 end

lib/active_admin/mongoid/helpers/collection.rb

@@ -0,0 +1,9 @@
+module ActiveAdmin
+  module Helpers
+    module Collection
+      def collection_size(collection=collection)
+        collection.count(true)
+      end
+    end
+  end
+end