The builtin WWW.responseHeaders property does not return all actual headers. E.g. if a request has multiple 'Set-Cookies' then the native will only return one of them. This is because the builtin function returns a Dictionary<string, string> which cannot have multiple keys with the same name. This extension to the WWW class makes it possible to …

WWWResponseHeaders.cs

using UnityEngine;
using System.Text.RegularExpressions;
using System.Reflection;
using System.Linq;
using System.Collections.Generic;

/// <summary>
/// Extensions for the WWW class
/// </summary>
public static class WWWExtension
{
    /// <summary>
    /// Get all response headers
    /// </summary>
    /// <param name="www">The WWW instance</param>
    /// <returns>All response headers for a request</returns>
    public static IEnumerable<KeyValuePair<string, string>> ResponseHeaders(this WWW www)
    {
        var propertyName = "responseHeadersString";
        var property = typeof(WWW).GetProperty(propertyName, BindingFlags.Public|BindingFlags.NonPublic|BindingFlags.Instance);
        if (property == null)
        {
            Debug.LogWarning("The Unity SDK might have changed so that the WWW class no longer has a " + propertyName + " property.");
            return null;
        }

        var headers = property.GetValue(www, null) as string;
        headers.Split('\n');
        var headerRegex = new Regex(@"(.*?):\s(.*)");
        var splitRegex = new Regex(": ");
        return headerRegex.Matches(headers).Cast<Match>().Select(x => splitRegex.Split(x.Value)).Select(x => new KeyValuePair<string, string>(x[0], x[1]));
    }
}