In django.db.models.base around line 304..305 there is a typo

Here, it reads:

```
    if isinstance(values, dict):
        for key, val in value.iteritems():
```

I guess that should read:

```
    if isinstance(values, dict):
        for key, val in value_s_.iteritems():
```

Please not that he additional underscores are provided for emphasis only.
