Bugfix for encode_with_override psych extension

The `coder` passed to` ActiveRecord::Base#encode_with` does not necessarily have
a `tag=` method (it may just be a `Hash`). This causes issues with other callers,
eg https://github.com/Shopify/identity_cache/blob/master/lib/identity_cache/query_api.rb#L140

Fixes https://github.com/collectiveidea/delayed_job/issues/435