[boost] [Convert] std::isspace requires unsigned #28

Hi,

boost\convert\base.hpp(112): warning C6330: 'const char' passed as _Param_(1) when 'unsigned char' is required in call to 'isspace'.

boost\convert\base.hpp(115): warning C6330: 'const char' passed as _Param_(1) when 'unsigned char' is required in call to 'isspace'.

https://github.com/boostorg/convert/issues/28 IMO the code should be fixed and it's not a documentation issue. What do you think? -- Olaf