Because if `c` is the letter 'I', and the current locale happens to be set to Turkish, then `tolower(c)` will return 'ı', not 'i'. If you are trying to lower-ca

Because if `c` is the letter 'I', and the current locale happens to be set to Turkish, then `tolower(c)` will return 'ı', not 'i'. If you are trying to lower-case an HTTP header name for the purpose of case-insensitive comparison, this is definitely not what you wanted. (And similar problems exist with several other locales; it's not just Turkish.)