For cookies, use cookie method instead of header method

[imefisto]

Description

fixes #13

How has this been tested?

You can reproduce the issue with this code:

<?php

define('PORT', 9502);

$http = new swoole_http_server("127.0.0.1", PORT);

$http->on("start", function ($server) {
    echo "Swoole http server is started at http://127.0.0.1:" . PORT . "\n";
});

$http->on("request", function ($request, $response) {
    $response->cookie("TestCookie1", "swoole1");
    $response->cookie("TestCookie2", "swoole2");
    $response->cookie("TestCookie3", "swoole3");
    $response->end();
});

$http->start();

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• [x ] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

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