Update src/Httpful/Response/Headers.php

If a header appears more than once, it must also be able to  be represented as a single header with a comma-separated list of values.  We transform accordingly.

Well, the cookies have commas on them on the date field so i'm using a double pipe for delimiter.

Set-Cookie: cookie_1=42de533-b4d2-4f28-8982-ccda555061a8; Domain=www.mydomain.com; Expires=Tue, 12-Feb-13 17:10:28 GMT; Path=/; HttpOnly 
Set-Cookie: cookie_2=somevalue; Domain=www.mydomain.com; Expires=Tue, 12-Feb-13 15:10:28 GMT; Path=/; HttpOnly