High five for the Heroku API designer who decided to use the Accept header to indicate the acceptable format and version of the response. This is the way to design HTTP APIs.

Instead, most companies put the version number in the URL and use special suffixes on the URL like ".json" to indicate these things. I can understand this practice from the standpoint of convenience. A human using cURL can't be bothered to type out an Accept header. But when building an API support should first be considered for explicit, programmatic API clients, not humans. Then you can slap on your human-friendly URL extensions, etc.

It's always easier to start out being explicit.