Review Core primitive types convert and validate methods

rimitive types have convert and validate methods.
convert method is called before setting the internal value, validate is called by the DocumentValidationService.

These methods implementation is not concistent across types: sometimes a value would be correctly converted but is not validated ok.
If core is able to perform conversion for some values, it should validate them ok.

If core wants to be more strict, then we can setup a boolean system value (nuxeo.conf prop) to change the behaviour