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.
Previously we'd only assign a response parser when a request came through
Action Dispatch integration tests. This made calls to `parsed_body` when a TestResponse
was manually instantiated — though own doing or perhaps from a framework — unintentionally
blow up because no parser was set at that time.
The response can lookup a parser entirely through its own ivars. Extract request encoder to
its own file and assume that a viable content type is present at TestResponse instantiation.
Since the default response parser is a no-op, making `parsed_body` equal to `body`, no
exceptions will be thrown.
