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

@@ -11,17 +11,22 @@ def not_sharded
     end
 
     def is_sharded? # rubocop:disable Naming/PredicateName
-      if self == ActiveRecord::Base
-        sharded != false && supports_sharding?
-      elsif self == base_class
-        if sharded.nil?
-          ActiveRecord::Base.is_sharded?
-        else
-          sharded != false
-        end
-      else
-        base_class.is_sharded?
-      end
+      # "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?
+                         ActiveRecord::Base.is_sharded?
+                       else
+                         sharded != false
+                       end
+                     else
+                       base_class.is_sharded?
+                     end
     end
 
     def on_replica_by_default?