Hydrator mutator isn't compatible with gzipped responses.

[juan131]

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Describe the bug

As it's described in the documentation, the hydrator mutator works by sending a POST request containing a JSON representation of AuthenticationSession to the URL you configure (it's also possible to customize the authentication headers if needed). However, the rest of headers to use on that POST request can't be customized.

It seems that Oathkeeper is accepting gzip-encoding but then it's unable to handle it.

Reproducing the bug

1. I have an Golang API that uses chi's as router and uses the "Compress" middleware (see https://github.com/go-chi/chi#core-middlewares), this middleware compress the HTTP responses using gzip when the requester adds the header Accept-Encoding: gzip. This API exposes an endpoint that accepts POST requests including a AuthenticationSession (as described in the docs).
2. Run Oathkeeper configuring a Hydrator mutator that uses the API mentioned above.
3. Test the Hydrator mutator by accessing a resources protected by Oathkeeper.
4. Oathkeeper is sending the POST accepting gzipped encoding, but then it's unable to decode the response (see relevant logs).

Relevant log output

{
    "error": {
      "message": "invalid character '\\x1f' looking for beginning of value"
    },
    (...)
    "level": "warning",
    "msg": "The mutation handler encountered an error",
    "mutation_handler": "hydrator",
    "reason_id": "mutation_handler_error",
    (...)
}

Relevant configuration

serve:
  mutators:
    hydrator:
      enabled: true
      config:
        api:
          url: https://MY-CUSTOM-API-WITH-GZIP-ENCODING-SUPPORT/

Version

v0.38.25-beta.1

On which operating system are you observing this issue?

Linux

In which environment are you deploying?

Kubernetes

Additional Context

If I disable the Compress middleware in my API, it works as expected.

[mvanderlee]

Same goes for brotli.