fix: incorrect handling of negative cookie.maxAge #1900

[ksw2000]

This PR fixes issue #1900

According to RFC6265, a max-age value of 0 or less is valid and is interpreted as forcing the cookie to expire immediately.

In the Go net/http package, they set Max-Age to 0 when the cookie.MaxAge < 0.

Before the fix

	net/http	fasthttp
maxAge = 0	unset	unset
maxAge > 0	Max-Age=maxAge	Max-Age=maxAge
maxAge < 0	Max-Age=0	unset

After the fix

	net/http	fasthttp
maxAge = 0	unset	unset
maxAge > 0	Max-Age=maxAge	Max-Age=maxAge
maxAge < 0	Max-Age=0	Max-Age=0

[erikdubbelboer]

Thanks! Didn't see this before I replied to the issue : 🙈