Inconsistency with MovingOLS min_periods
I'm not sure if this is a bug or a typo in the documentation but for the `min_periods` parameter in MovingOLS the documentation claims

```
min_periods : int
       Threshold of non-null data points to require.
       If None, defaults to size of window.
```

However this does not seem to be the case? From line 640-648 we have

```
    if self._is_rolling:
        if window is None:
            raise AssertionError("Must specify window.")
        if min_periods is None:
            min_periods = window
    else:
        window = len(self._x)
        if min_periods is None:
            min_periods = 1
```

So this is not true for expanding window MovingOLS? Would it be appropriate to update the documentation to reflect this?
