Incorrect handling of negative MaxAge cookie attribute

[milesich]

Currently cookies with negative Max-Age attribute will throw en exception. But according to RFC6265:

If the first character of the attribute-value is not a DIGIT or a "-" character, ignore the cookie-av.

I found this problem when I was trying to import a feed using Zend\Feed\Reader\Reader::import and in the feed headers was this cookie:

Set-Cookie: wpSGCacheBypass=0; expires=Fri, 15-Apr-2016 07:10:31 GMT; Max-Age=-3600; path=/

[Xerkus]

Fixed in #120