Weird hack to handle different params hash

For some reason the opener-webservice (v2.0.0) is sending the params hash with `:body => { :input => ""...` so this was getting wrapped in an extra `:body =>` here. In order to fix this I've added this weird hack.

This is a fix for https://github.com/opener-project/callback-handler/issues/1 . This may not be your preferred way of fixing this issue, but I've created the PR since I'd already done it in my fork.