code: public RequestBody body(Serializer serializer) throws IOException {
          if (serializer == null) {
              serializer = Serializer.defaultSerializer();
          }
  
          ByteArrayOutputStream stream = new ByteArrayOutputStream(4096);
          serializer.serializeParams(stream, this);
          return RequestBody.create(JSON_MEDIA_TYPE, stream.toByteArray());
      }
docs: Returns the {@link RequestBody} to send with the HTTP request. The default implementation serializes
  the instance into JSON format and builds an HTTP {@link RequestBody} containing the serialized data.
  
  @param serializer The {@link Serializer} to use to serialize parameter data.
  Or `null` to use the default serializer.
  @return An {@link RequestBody} to send with the HTTP request.
  @throws IOException on serialization errors.