Session cookie survives (maxAge + zero value bug)
I'm using `koa-session` with params:
```
{
      key:     'sid',
      prefix:  'sess:',
      httpOnly:  true,
      path:      '/',
      overwrite: true,
      signed:    false,
      maxAge:    3600 * 4 * 1e3, 
      rolling: true
}
```

Session removal does this: `ctx.cookies.set(key, '', opts)`.

Unfortunately, in "cookies" module, `toHeader` has this:

```
if (this.maxAge) this.expires = new Date(Date.now() + this.maxAge);
```

So no value (cookie deletion) actually leads to empty cookie with future expiration.

