Use php://input stream instead of building our own IPN validation array

From what I understand, PayPal just verifies that the data we send to them matches the data they sent to us. Using $_POST and building the $variables array, we're not really adding or removing anything. However, the loop to build the array does not work with some PayPal APIs that have nested arrays such as Adaptive Payments. Why not just send them back the raw data they sent to us using php://input?

Patch incoming.