Assuming all JSON payloads are application/json is wrong

[inadarei]

This is regarding: https://github.com/mikeal/r2/blob/master/index.js#L73

If current code sees that you are passing json it overrides content-type header to be set to application/json. But application/json is not the only JSON media type. There're plenty of others:

1. HAL http://stateless.co/hal_specification.html
2. JSON API http://jsonapi.org/
3. Siren https://github.com/kevinswiber/siren
4. Hyper http://hyperjson.io/

to name just a few

[bcomnes]

Probably just make the opts.headers override this.