Fix the va_args order of http header

[beef9999]

The current HTTP header has a bug of wrong key value order.
This is the POC. It will print v first, and then k, which is obviously wrong.

#include <iostream>
#include <string>
#include <map>
#include <string>
#include <cstdarg>

std::map<std::string, std::string> common_header;

void add_header(va_list args) {
    common_header[va_arg(args, const char*)] = va_arg(args, const char*);
}

void f(int a, ...) {
    va_list args;
    va_start(args, a);
    add_header(args);
    va_end(args);
}

int main() {
    f(1, "k", "v");
    for (auto& i: common_header) {
        std::cout << i.first << std::endl;
        std::cout << i.second << std::endl;
    }
}