HttpRequest getHeaders() returns only last occurrence of multiple header fields with the same name

Steps to reproduce:

• Create a script item to retrive an url that sets multiple cookies on request ex:

req = new HttpRequest();
resp=req.post("http://{$BASE_IP}/lua/api",payload);
return JSON.stringify(req.getHeaders()['Set-Cookie']);

Result
Only last 'Set-Cookie' header is returned

Expected
An array or some other way to retrive all 'Set-Cookie' headers