Windowing fails on read-only arrays
#### Code Sample, a copy-pastable example if possible

```python
>>> arr = np.array([1, 3, np.nan, 3, 5])
>>> arr.setflags(write=False)
>>> pd.Series(arr).rolling(2).mean().values
---------------------------------------------------------------------------
ValueError: assignment destination is read-only
```

#### Expected Output
```py
>>> pd.Series(arr).rolling(2).mean().values
array([nan,  2., nan, nan,  4.])
```
#### Problem description

https://github.com/pandas-dev/pandas/pull/27391#discussion_r309941141 missed a copy call before writing into array, which is user's data and which may be read-only.

#### Output of ``pd.show_versions()``

pandas 0.25.0
