Need a way to post raw payload

[pandafox]

https://github.com/ravenjohn/cuddle/blob/master/src/Request.js#L302

https://support.tipalti.com/Content/Topics/Development/IPNs/ipn_protocol.htm
Needed for IPN validation, requires sending back exactly the content. JSON.stringify doesn't work because

1. Request.stringify may not keep the right order of the fields
2. JSON.stringify adds unwanted quotes around the raw data