Aws elb parser fix

Lots of little fixes for test failures I discovered when running with mocks disabled locally. I also tried to update the mocks to mirror AWS behavior as closely as possible.

I added a `DEBUG_RESPONSE` environment variable to allow me to see the xml response for all requests. I tried using the `EXCON_DEBUG` env var at first but the body was always empty even when I saw content with tcpdump. My guess is that there may be a bug in excon around displaying an IO object that has already been read by the parser and has no more content to show, hence the empty body.