fix: avoid reusing instances for classes reloaded in development

This only affects development, when using threaded servers like Puma
AND with concurrency enabled.

The bug manifests as serializers "not being updated", and the cause is
that  even though the class itself is being reloaded correctly, the
`instance` method will a previously cached instance of a previous
version of the class.

By using `object_id`, if the class was reloaded, a new instance will be
created.

lib/oj_serializers/serializer.rb

@@ -448,7 +448,7 @@ def instance
     # Internal: Cache key to set a thread-local instance.
     def instance_key
       unless defined?(@instance_key)
-        @instance_key = "#{name.underscore}_instance".to_sym
+        @instance_key = "#{name.underscore}_instance_#{object_id}".to_sym
         # We take advantage of the fact that this method will always be called
         # before instantiating a serializer to define the write_to_json method.
         class_eval(write_to_json_body)