Fix Jackson SPI

[SentryMan]

resolves #442

[mechite]

debatably could be worth adding test, e.g.,

class JacksonAdapterFactoryTest {

  @Test
  void serviceLoader_jacksonAdapter() {
    Jsonb jsonb = Jsonb.instance();
    var input = new ByteArrayInputStream(new byte[0]);
    var output = new ByteArrayOutputStream();

    assertThat(jsonb.reader(input)).isInstanceOf(JacksonReader.class);
    assertThat(jsonb.writer(output)).isInstanceOf(JacksonWriter.class);
  }
}