Uses deprecated method for passing credentials

[davidcollie]

A recent commit 28b2368 updated the authentication to pass the authentication parameters in the body.

However, post and get requests still pass the user_key and api_key as query params instead of in the Authorization: header for a GET request OR in the body of the POST request.

http://developer.pardot.com/#using-the-api

Pardot's documentation shows samples of what they expect going forward.

I believe this is why API requests in Pardot's API usage dashboard are still showing Uses Deprecated Auth when using the latest version of the ruby-pardot gem.