Cache the is_sharded? status of the model

Sets self.sharded depending on the superclass, if it is not otherwise
set.

lib/active_record_shards/model.rb

@@ -11,7 +11,12 @@ def not_sharded
     end
 
     def is_sharded? # rubocop:disable Naming/PredicateName
-      if self == ActiveRecord::Base
+      # "sharded" here means self.sharded, but actually writing "self.sharded"
+      # doesn't work until Ruby 2.7 (and this gem currently supports 2.6).
+      return sharded unless sharded.nil?
+
+      # Despite self.sharded not working, self.sharded= _DOES_ work.
+      self.sharded = if self == ActiveRecord::Base
         sharded != false && supports_sharding?
       elsif self == base_class
         if sharded.nil?