captioned image was incorrectly setting its id property.

There were 2 problems.

* Since there are no private properties, if the base class sets it, the derived class needs to be careful when using the property, in this case the id property belongs to the base Image class and the CaptionedImage needs to create a new property captioned_id.
* It was using the undefined config.id instead of this.captioned_id to set the id property in the config passed to its base class.