Method buf_start_with

external/cpp-httplib/httplib.h:3994–3996

            std::string buf_head(size_t l) const { return buf_.substr(buf_spos_, l); }

            bool buf_start_with(const std::string& s) const {
                return start_with(buf_, buf_spos_, buf_epos_, s);
            }

            size_t buf_find(const std::string& s) const {
                auto c = s.front();