fix: delegate issue for text element in xml

[HassanAkbar]

Fixing issue when using delegate option with custom models.

fix #57

[ronaldtse]

LGTM. Do we need to update readme?

[HassanAkbar]

Do we need to update readme?

@ronaldtse I think we don't need to update the readme.

This is the example that could have failed with custom model. So I think the readme already includes this.

  map_content to: :name_of_content, with: {
    to: :method_name_to_serialize,
    from: :method_name_to_deserialize
  }