C++: Fix MSVC assert in std::isspace()

...when dealing with UTF8 bytes.

std::isspace() expects unsigned char, not char.

Change-Id: I3f9b5e347d79cf94015cc99f8797d5feab406151
Reviewed-by: Orgad Shaneh <orgads@gmail.com>
Reviewed-by: Marco Bubke <marco.bubke@theqtcompany.com>