id,node_id,number,title,user,state,locked,assignee,milestone,comments,created_at,updated_at,closed_at,author_association,active_lock_reason,draft,pull_request,body,reactions,performed_via_github_app,state_reason,repo,type
462424005,MDU6SXNzdWU0NjI0MjQwMDU=,3066,xarray rolling does not match pandas when using min_periods and reduce,4903456,open,0,,,2,2019-06-30T16:44:09Z,2021-02-18T21:41:57Z,,NONE,,,,"#### MCVE Code Sample

**MCVE**
```python

import numpy as np
import pandas as pd
import xarray

def custom(x, axis=0):
    return np.mean(x, axis)

d = pd.DataFrame(np.random.rand(100,3))
r = d.rolling(10, min_periods=5).apply(custom)
print(r.iloc[0:10,:])

xd = d.to_xarray().to_array()
r = xd.rolling(index=10, min_periods=5).reduce(custom)
print(r[:,0:10])
r = xd.rolling(index=10, min_periods=1).reduce(custom)
print(r[:,0:10])

```

#### Problem Description

I am applying a custom function on rolling windows with specific `min_periods`. The output of `pandas..rolling.apply` matches what I expect; however, the output of `xarray..rolling.reduce` doesn't seem to take `min_periods` into account.

#### Expected Output and Actual Output
```
          0         1         2
0       NaN       NaN       NaN
1       NaN       NaN       NaN
2       NaN       NaN       NaN
3       NaN       NaN       NaN
4  0.632168  0.523669  0.543643
5  0.558694  0.565781  0.481204
6  0.559343  0.541787  0.415490
7  0.613457  0.554888  0.398999
8  0.579552  0.496799  0.397681
9  0.562591  0.525096  0.416461
<xarray.DataArray (variable: 3, index: 10)>
array([[     nan,      nan,      nan,      nan,      nan,      nan,      nan,
             nan,      nan, 0.562591],
       [     nan,      nan,      nan,      nan,      nan,      nan,      nan,
             nan,      nan, 0.525096],
       [     nan,      nan,      nan,      nan,      nan,      nan,      nan,
             nan,      nan, 0.416461]])
Coordinates:
  * index     (index) int64 0 1 2 3 4 5 6 7 8 9
  * variable  (variable) int64 0 1 2
<xarray.DataArray (variable: 3, index: 10)>
array([[     nan,      nan,      nan,      nan,      nan,      nan,      nan,
             nan,      nan, 0.562591],
       [     nan,      nan,      nan,      nan,      nan,      nan,      nan,
             nan,      nan, 0.525096],
       [     nan,      nan,      nan,      nan,      nan,      nan,      nan,
             nan,      nan, 0.416461]])
Coordinates:
  * index     (index) int64 0 1 2 3 4 5 6 7 8 9
  * variable  (variable) int64 0 1 2
```

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.8 |Anaconda, Inc.| (default, Dec 29 2018, 19:04:46)
[GCC 4.2.1 Compatible Clang 4.0.1 (tags/RELEASE_401/final)]
python-bits: 64
OS: Darwin
OS-release: 18.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8
libhdf5: 1.10.4
libnetcdf: 4.6.1

xarray: 0.12.1
pandas: 0.24.2
numpy: 1.16.4
scipy: 1.2.1
netCDF4: 1.4.2
pydap: None
h5netcdf: None
h5py: 2.9.0
Nio: None
zarr: None
cftime: 1.0.3.4
nc_time_axis: None
PseudonetCDF: None
rasterio: None
cfgrib: None
iris: None
bottleneck: 1.2.1
dask: 2.0.0
distributed: 2.0.1
matplotlib: 3.1.0
cartopy: None
seaborn: 0.9.0
setuptools: 41.0.1
pip: 19.1.1
conda: None
pytest: None
IPython: 7.5.0
sphinx: None

</details>
","{""url"": ""https://api.github.com/repos/pydata/xarray/issues/3066/reactions"", ""total_count"": 0, ""+1"": 0, ""-1"": 0, ""laugh"": 0, ""hooray"": 0, ""confused"": 0, ""heart"": 0, ""rocket"": 0, ""eyes"": 0}",,,13221727,issue