Fix leading semicolon when stringifying cookies with skipped values

[saripovdenis]

Fixes stringifyCookie output when the first cookie entry has an undefined value.

Previously, separators were based on the loop index. If the first key was skipped, the next emitted cookie still got a leading ; :

stringifyCookie({ a: undefined, b: "2" })
// before: "; b=2"
// after: "b=2"

This changes separator insertion to check whether output has already been written.

Test added for the skipped-first-entry case.

Validation:

• npm run specs -- src/stringify-cookie.spec.ts
• npm test