Fixed serialization of empty body

[wkornewald]

Subsystem
Client: JsonFeature/JsonSerializer

Motivation
When using JsonFeature the empty body (e.g. a get() request without a body) gets serialized to {} instead of an empty string. This causes problems with some backends which expect the body to be empty.

For more context, let's assume we define an HttpClient with JsonFeature and a default Content-Type header like this:

val client = HttpClient(...) {
    defaultRequest {
        contentType(ContentType.Application.Json)
    }
    install(JsonFeature) {
        // ...
    }
}

This gives us some more convenience because we don't need to add the Content-Type header to each post()/put() call. However, now every request where we don't assign a body (e.g. a simple get("https://...")) gets turned into a request containing {} in the body and having a Content-Type: application/json header.

This causes some backends to return an error because they expect an empty body for get() requests. Often, the backend isn't under our control, so we have to comply with the usual expection: No body in get() should mean no body in the resulting HTTP request.

Solution
With this PR, JsonFeature simply passes through EmptyContent. I think this really is the most sensible behavior, mapping the intent 1:1. It also removes the Content-Type header because there really is no content.

If someone wants to send an empty {} then that's still possible by explicitly setting e.g. body = Unit (or any serializable object without attributes). At least, now it's explicit.

If someone wants to send an empty body, but with Content-Type set (for obscure backend requirements), that's also still possible by customizing the serializer and using a special object as a marker.

[wkornewald]

@e5l Since I can't add reviewers, could you please have a look at this fix? :)

[wkornewald]

I've updated the solution to directly deal with this in JsonFeature. At first I thought it would be better to leave this decision to the serializer (because who knows how some serialization formats represent "no content"), but in the end that's an invitation for inconsistency across serializers and thus it's too error-prone (I almost forgot about KotlinxSerializer because it's not covered by JsonTest).

[e5l]

Hi @wkornewald, thanks for the PR.

LGTM! Just needs the minor fix to get unified behavior.

[wkornewald]

@e5l You mean, like in the new commit I just pushed?

[e5l]

LGTM, merged. Thanks for the fast response!