StringUtils: fix TrimLeft and TrimRight functions with UTF-8 strings
At least on Win32 TrimLeft and TrimRight failed with UTF-8 strings. When dereferencing std::string::iterator through not(ptf_fun(isspace())) character converted to int as signed char (isspace declared as "int isspace(int)").
Merge pull request #3403 from Karlson2k/string_utils_fix_01

StringUtils: fix TrimLeft and TrimRight functions with UTF-8 strings