@ParameterizedTest
    @ArgumentsSource(ServiceProvider.class)
    void nullContentType(Service service) throws Exception {
        final Response<Void> response = service.postCustomContentType(null).get();
        assertThat(response.code()).isEqualTo(200);
    }
Tests https://github.com/line/armeria/pull/386
retrofit2/src/test/java/com/linecorp/armeria/client/retrofit2/ArmeriaCallFactoryTest.java