std::tolower is undefined if the argument's value is neither representable as unsigned char nor equal to EOF (#761)

Co-authored-by: taoxu <taoxu@bilibili.com>

httplib.h

@@ -271,7 +271,7 @@ struct ci {
   bool operator()(const std::string &s1, const std::string &s2) const {
     return std::lexicographical_compare(
         s1.begin(), s1.end(), s2.begin(), s2.end(),
-        [](char c1, char c2) { return ::tolower(c1) < ::tolower(c2); });
+        [](unsigned char c1, unsigned char c2) { return ::tolower(c1) < ::tolower(c2); });
   }
 };