Remove #set_form_data from #send_back request

@tc ,
First, thank you for this gem. I have been using it on a project for a while and it has been great.

I noticed a little while back that when updated an unrelated get, it in turn updated the version of Rack. After that, all my responses from Paypal's IPN were 'INVALID.' After some digging I was able to determine that the #send_back method in the ipn_notification was jumbling the response back to Paypal. Paypal's IPN docs say explicitly that the return value must be identical to the one that was sent.

This solution solution should solve the issue by encoding the response properly while keeping the hash in tact (and in the same order).