utility to combine PaymentTransactionInfoPlugins

Maybe the pull request can be simplified to hide internal changes that aren't interesting after the fact
You could refactor it with something like:

```
[:kb_payment_id, :kb_transaction_payment_id, :transaction_type, :amount, :currency, :created_date, :effective_date, :gateway_error, :gateway_error_code, :first_payment_reference_id, :second_payment_reference_id].each do |attr|
  t3.send("#{attr.to_s}=") = t1.send(attr.to_s)
end
```

This has the advantage of being able to quickly spot missing attributes declared here: https://github.com/killbill/killbill-plugin-framework-ruby/blob/master/lib/killbill/gen/plugin-api/payment_transaction_info_plugin.rb#L36