Some events are being dropped

I am using the lib for a project of mine, and I noticed some events are being dropped. I added printing calls to the official Mixpanel lib (that MPA uses) and used the following code:

```
mp.track(str(request.user.pk), "changed settings")
mp.people_set(str(user.pk), {
"$email": user.email,
"$name": user.name,
"$ip": request.META.get('HTTP_X_FORWARDED_FOR', request.META.get('REMOTE_ADDR')).split(',')[-1].strip(),
"username": user.username,
})
mp.track(str(request.user.pk), "changed settings 2")
```

What came out was:

```
web_1 | {'data': b'[{"properties":{"distinct_id":"21","token":"decoded this from base64 to redact the token","time":1508248513,"$lib_version":"4.3.2","mp_lib":"python"},"event":"changed settings"}]'
, 'verbose': 1, 'ip': 0}
web_1 | 172.26.0.5 - - [17/Oct/2017 13:55:13] "POST /settings/ HTTP/1.1" 302 -
```

And that's it. Even after minutes, nothing else is sent. I'm using `runserver_plus` to run this, but it also happens on production with uWSGI. You will notice that only the first event got sent, and all subsequent events are lost.

Start by tracing the mp.track and mp.people_set calls in the reported sequence, comparing the first event with the later calls. Reproduce it under runserver_plus and uWSGI, then verify that subsequent events are sent rather than lost.

Scritto dal modello di indicizzazione a partire dal testo della issue.