@Test
public void should_post_A() throws Exception {

    HttpTestClient httpTestClient = HttpTestClient.withBaseUrl("http://localhost:8080");
    httpTestClient = server.client();
    HttpRequest httpRequest = httpTestClient.authenticatedAs("admin")
            .POST("/api/polymorphic")
            .contentType("application/json")
            .send("{\"@class\":\".PolymorphicResource$A\",\"a\":\"a3\"}");
    assertThat(httpRequest.code()).isEqualTo(200);
    assertThat(httpRequest.body().trim()).isEqualTo("{\n" +
            "  \"@class\" : \".PolymorphicResource$A\",\n" +
            "  \"a\" : \"a3\"\n" +
            "}");
}