Incorrect handling of Cookie.SetMaxAge with value 0 or less

[anthonydresser]

Per the spec that was reference during the cookie max-age implementation (#184), https://datatracker.ietf.org/doc/html/rfc6265#section-5.2.2, a max-age value of 0 or less is valid and is interpreted as forcing the cookie to expire immediately.

In the current implementation, setting max-age to 0 or less causes the max age attribute to be completely ignored, making it impossible to delete a cookie via the max-age attribute. https://github.com/valyala/fasthttp/blob/master/cookie.go#L281

Given 0 is the default value of int in go, probably doesn't make sense to support max-age=0, but supporting setting maxAge to <0 should be possible by changing the maxAge > 0 to maxAge != 0.

	if c.maxAge != 0 {
		dst = append(dst, ';', ' ')
		dst = append(dst, strCookieMaxAge...)
		dst = append(dst, '=')
		dst = AppendUint(dst, c.maxAge)
	} else if !c.expire.IsZero() {
		c.bufV = AppendHTTPDate(c.bufV[:0], c.expire)
		dst = append(dst, ';', ' ')
		dst = append(dst, strCookieExpires...)
		dst = append(dst, '=')
		dst = append(dst, c.bufV...)
	}

Alternatively, the maxAge property could be changed to a *int to support setting max-age to 0 as well and check for nil during the creation of the cookie header.

[ksw2000]

In golang net/http package. They set Max-Age to 0 when the c.MaxAge < 0.

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

https://github.com/golang/go/blob/2eac154b1c8b51d05fa5b110ae065d3610a61e06/src/net/http/cookie.go#L265-L270

if c.MaxAge > 0 {
	b.WriteString("; Max-Age=")
	b.Write(strconv.AppendInt(buf[:0], int64(c.MaxAge), 10))
} else if c.MaxAge < 0 {
	b.WriteString("; Max-Age=0")
}

[erikdubbelboer]

I think just supporting <0 and converting it to 0 like net/http does is best. Can you make a pull request?