text: Title: Include `Message.html` in JSON results only if it is `True`
  Body: Currently `robot.result.Message` is serialized to JSON so that the `html` attribute is always included. Including it only if it is `True` makes the resulting JSON data a bit smaller. That is also consistent with other similar attributes that are serialized to JSON only when their value differs from the default value.
  
  Robot itself can load JSON where `html` is included, regardless is it `True` or `False`, and where it is omitted (in which case it is considered to be `False`) so in that regard the change is fully backwards compatible. External tools processing JSON results need to be updated if they expect `html` to be always set, though. JSON result support is so new that I don't expect many tools to be affected, so the change ought to be fine even in a non-major release. There are likely more tools like that in the future, especially if we get #3423 done, and delaying this change could thus cause bigger problems later.