Use the full class name for the JSON coder, as there may be another

`JSON` constant defined.

For example when using the `representable` gem: https://github.com/trailblazer/representable/issues/224

activestorage/app/models/active_storage/blob.rb

@@ -19,7 +19,7 @@ class ActiveStorage::Blob < ActiveRecord::Base
   self.table_name = "active_storage_blobs"
 
   has_secure_token :key
-  store :metadata, accessors: [ :analyzed, :identified ], coder: JSON
+  store :metadata, accessors: [ :analyzed, :identified ], coder: ActiveRecord::Coders::JSON
 
   class_attribute :service