More win in serialization scenarios wrt curr.cult.
If the object will be serialized via ToString, it will be checked
whether the object implements IFormattable and then a CultureInvariant
ToString overload is called.
Tests for the IFormattable usage
This commit is separated from the implementation
as it is unclear whether the Rect class is available in all environments
Merge pull request #357 from flq/culture_issue

Culture issue