Instead of appending a format to the request, it's much better
to just pass a more appropriate accept header. Rails will figure
out the format from that instead.

This allows devs to use `:as` on routes that don't have a format.

Introduce an `IdentityEncoder` to avoid `if request_encoder`,
essentially a better version of the purpose of the `WWWFormEncoder`.
One that makes conceptual sense on GET requests too.

Fixes #27144.