code: public <T> T decodeWithChildContext(final Decoder<T> decoder, final BsonReader reader) {
          return decoder.decode(reader, DEFAULT_CONTEXT);
      }
docs: Creates a child context and then deserializes using the reader.
  
  @param decoder the decoder to decode with
  @param reader the reader to decode to
  @param <T> the type of the decoder
  @return the decoded value
  @since 3.5