@Test
  public void undefined() throws IOException {
    JsonReader reader = Jsons.readerAt(new BsonUndefined());
    check(reader.peek()).is(JsonToken.NULL);
    reader.nextNull();
  }
Validate type conversion between BSON and gson-JSON ({@link com.google.gson.stream.JsonToken}).
mongo/test/org/immutables/mongo/bson4gson/TypeConversionTest.java