fix(cookies): keep an Expires of 0 when serializing a cookie

[NgoQuocViet2001]

Problem

stringify tests expires for truthiness:

if (cookie.expires && cookie.expires.toString() !== 'Invalid Date') {
  out.push(`Expires=${toIMFDate(cookie.expires)}`)
}

expires is typed Date | number, and a numeric 0 is the Unix epoch — the canonical way to say expire this cookie now. It is falsy, so the attribute is silently dropped.

The Max-Age branch a few lines above already avoids exactly this:

if (typeof cookie.maxAge === 'number') {   // 0 is a real value

Effect

expires: 0             -> "a=b"                                            <- attribute lost
expires: new Date(0)   -> "a=b; Expires=Thu, 01 Jan 1970 00:00:00 GMT"
expires: 1000          -> "a=b; Expires=Thu, 01 Jan 1970 00:00:01 GMT"
maxAge: 0              -> "a=b; Max-Age=0"

Only the numeric-zero case, and only for expires. No error is raised — a caller asking for immediate expiry gets a session cookie instead, which is the unsafe direction for the usual purpose (clearing a cookie on logout).

Fix

Treat null/undefined as absent instead of treating every falsy value that way. After the change:

expires: 0            -> "a=b; Expires=Thu, 01 Jan 1970 00:00:00 GMT"
expires: null         -> "a=b"
no expires            -> "a=b"
expires: new Date(0)  -> "a=b; Expires=Thu, 01 Jan 1970 00:00:00 GMT"
expires: invalid Date -> "a=b"

Test plan

• Added two cases to test/cookie/cookies.js beside the existing numeric-expires test: expires: 0 serializes the epoch, and expires: null still omits the attribute.
• Ran: node --test test/cookie/*.js → 93 passing.
• Checked: reverting only util.js fails the new test with 'Space=Cat' == 'Space=Cat; Expires=Thu, 01 Jan 1970 00:00:00 GMT'.
• Ran: npx standard on both touched files → clean.