Regenerated session is re-saved even if not modified since save

PR #849 attempted to fix resaving an already-saved new session at the end of the request, but there's one corner case it missed. If `session.regenerate()` has been called before `session.save()`, then the request is still saved again at the end of the request.

This can actually lead to a race condition: if another request modifies the session after `session.save()` but before the end of the first request, then those modifications get overwritten when the first request ends. (And yes, this can happen in real world...)

How to reproduce: This code prints "saving" twice for one request (also verified by adding logging inside express-session)

```
const MemoryStore = require('express-session/session/memory')

const _set = MemoryStore.prototype.set
MemoryStore.prototype.set = function set() {
console.log('saving')
_set.apply(this, arguments)
}

const express = require('express')
const session = require('express-session')
const app = express()

app.use(session({
secret: 'cat',
resave: false,
saveUninitialized: false
}))

app.get('/', (req, res) => {
req.session.regenerate(() => {
req.session.value = 'foo'
req.session.save(() => {
res.send('hello')
})
})
})

app.listen(3000)
```