Simplify is_sharded? calculation further

Instead of checking class inheritance we can just call a class method. This way
we can define it to be false for ActiveRecord::Base and override it in sharded
subclasses. This also gives us the possibility of using a module (or just plain
defining the method in any random class).

lib/active_record_shards/model.rb

@@ -10,7 +10,7 @@ def not_sharded
     end
 
     def is_sharded? # rubocop:disable Naming/PredicateName
-      self <= ActiveRecordShards::ShardedModel
+      false
     end
 
     def on_slave_by_default?

lib/active_record_shards/sharded_model.rb

@@ -1,5 +1,15 @@
 module ActiveRecordShards
+  module Ext
+    module ShardedModel
+      def is_sharded? # rubocop:disable Naming/PredicateName
+        true
+      end
+    end
+  end
+
   class ShardedModel < ActiveRecord::Base
     self.abstract_class = true
+
+    extend ActiveRecordShards::Ext::ShardedModel
   end
 end