Faraday no longer returns nil response bodies for net/http, so handle it better

https://github.com/lostisland/faraday/commit/f41ffaabb72d3700338296c79a2084880e6a9843

We got around it by having parse_json always return nil for non application/json responses,
but some people use the API client to download actual files, so we need to let those through.