cookie parser: handle 'secure='

There are two keywords in cookie headers that don't follow the regular
name=value style: secure and httponly. Still we must support that they
are written like 'secure=' and then treat them as if they were written
'secure'. Test case 31 was much extended by Rob Ward to test this.

Bug: http://curl.haxx.se/bug/view.cgi?id=3349227
Reported by: "gnombat"