perf: 2x faster hash stringify

[cesco69]

With huge session object the hash method become slower because iterate on each key/value of the object, just for remove the cookie property on the first level!

This PR make hash method 2/3x faster!

Online benchmark

This PR use the spread operator available on Node.js >= 8.3

if we need to support old NodeJS, I can introduce a new options for cusom hash function, eg:

app.use(session({
  hash: function(sess) {
    // serialize
    const { cookie, ...sessWithoutCookie } = sess;
    const str = JSON.stringify(sessWithoutCookie);

    // hash
    return crypto
      .createHash('sha1')
      .update(str, 'utf8')
      .digest('hex')
  },
  secret: 'keyboard cat'
}))

In this case, close this PR and merge this one #1035

Side note: this new options, also can close #990