support for faraday 0.9

This is more of a feature request than a pull request as I don't pretend to know a lot about rabbitmq or rabbitmq_http_api_client, but I've been chasing dependencies in my current work and I needed to get faraday up to 0.9.x, but got stuck on rabbitmq_http_api_client.

I *think* this is all that's needed to work with faraday 0.9 and I've tested with both 0.8.9 and 0.9.0 and this change seems backwards compatible (I get a handful of test failures, but they're the same with 0.8.9 and 0.9.0, and look like what I'm seeing in Travis for this project, so it's at least consistent and not noticeably regressing).

If we could loosen the faraday version requirement here, it would make my life a lot easier.  Thanks for the project!