Ktor test dostaje 415

Próbuję sobie ogarnąć Ktor, wraz z testami i trochę trace pomysły:
mam tak postawiony serwer:

fun main() {
    embeddedServer(Netty, port = 8080, host = "localhost") {
        install(ContentNegotiation) {
            json()
        }
        rootModule()
        personsCrud()
    }.start(wait = true)
}

fun Application.rootModule() {

    // Starting point for a Ktor app:
    routing {
        get("/") {
            call.respondText("Hello World!")
        }
    }
    routing {
    }
}

fun Application.personsCrud() {
    routing {
        post("/person") {
            val person = call.receive<Person>()
            println("${person.firstName} ${person.lastName}")
            call.respond(PostResponse(1))
        }
    }
}

I fajnie, działa jak sobie uderzę takim zapytaniem:

POST http://localhost:8080/person
Content-Type: application/json

{"firstName":"John", "lastName":"Doe"}


HTTP/1.1 200 OK
Content-Length: 8
Content-Type: application/json
Connection: keep-alive

{
  "id": 1
}

Ale jeżeli próbuję zrobić to z poziomu testu:

    @Test
    fun testPostPerson() = testApplication {
        application {
            personsCrud()
        }

        val client = createClient {
            install(ContentNegotiation) {
                json(Json {
                    prettyPrint = true
                    isLenient = true
                })
            }
        }

        client.post("/person") {
            contentType(ContentType.Application.Json)
            setBody(Person("John", "Doe"))
        }
            .apply {
                assertEquals(HttpStatusCode.OK, this.status)
            }
    }

expected:<200 OK> but was:<415 Unsupported Media Type>

Ma ktoś jakiś pomysł?

application {
    personsCrud()
}

brakuje

install(ContentNegotiation) {
    json()
}