text: Title: Remote: Enhance `datetime`, `date` and `timedelta` conversion
  Body: XML-RPC used by the Remote API supports only [some data types](https://en.wikipedia.org/wiki/XML-RPC#Data_types). Currently the Remote library converts everything it doesn't recognize to strings, but there are several issues:
  
  - Some types, at least `datetime`, are converted to strings even though XML-RPC actually supports them natively.
  - Some types are converted to strings even though some other type would be better. For example, `timedelta` should probably be converted to a float by using `timedelta.total_seconds()`. Also `date` could be converted to `datetime` that then would be handled automatically.
  - It's possible that with some types we should do some more formatting than just call `str()`.
  
  The main motivation for this issues is making it easier for remote servers to convert values not supported by XML-RPC back to appropriate types. As an example, see robotframework/PythonRemoteServer#84.
  
  In practice we need to go through all types that Robot's argument conversion supports and see what's the best way to handle them.