BUG: rolling count does not respect the default behavior of min_periods
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

Per the documentation:

> For a window that is specified by an offset, min_periods will default to 1. Otherwise, min_periods will default to the size of the window.

Therefore these two statements should be equal

```
In [4]: pd.__version__
Out[4]: '1.2.0.dev0+160.gd90b73bde'

In [5]: pd.Series([np.nan] * 4).rolling(2).count()
Out[5]:
0    0.0
1    0.0
2    0.0
3    0.0
dtype: float64

In [6]: pd.Series([np.nan] * 4).rolling(2, min_periods=2).count()
Out[6]:
0    NaN
1    0.0
2    0.0
3    0.0
dtype: float64
```