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
630062936,MDU6SXNzdWU2MzAwNjI5MzY=,4120,coarsen deletes attrs on original object,2448579,closed,0,,,13,2020-06-03T14:50:16Z,2020-09-09T19:30:42Z,2020-09-09T18:56:36Z,MEMBER,,,,"<!-- A short summary of the issue, if appropriate -->


#### MCVE Code Sample
<!-- In order for the maintainers to efficiently understand and prioritize issues, we ask you post a ""Minimal, Complete and Verifiable Example"" (MCVE): http://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports -->

```python
# Your code here
import xarray as xr

ds = xr.tutorial.load_dataset(""air_temperature"")
ds2 = xr.tutorial.load_dataset(""air_temperature"")

xr.testing.assert_identical(ds, ds2) # passes
ds.coarsen(lat=5).mean()
xr.testing.assert_identical(ds, ds2) # fails
```

Bug:  `coarsen` reductions are modifying the original dataset by deleting all attributes

```
AssertionErrorTraceback (most recent call last)
<ipython-input-21-b0a179f01c99> in <module>
     48 xr.testing.assert_identical(ds, ds2)
     49 ds.coarsen(lat=5).mean()
---> 50 xr.testing.assert_identical(ds, ds2)

~/work/python/xarray/xarray/testing.py in assert_identical(a, b)
     87         assert a.identical(b), formatting.diff_array_repr(a, b, ""identical"")
     88     elif isinstance(a, (Dataset, Variable)):
---> 89         assert a.identical(b), formatting.diff_dataset_repr(a, b, ""identical"")
     90     else:
     91         raise TypeError(""{} not supported by assertion comparison"".format(type(a)))

AssertionError: Left and right Dataset objects are not identical

Differing coordinates:
L * lat      (lat) float32 75.0 72.5 70.0 67.5 65.0 ... 25.0 22.5 20.0 17.5 15.0
R * lat      (lat) float32 75.0 72.5 70.0 67.5 65.0 ... 25.0 22.5 20.0 17.5 15.0
    standard_name: latitude
    long_name: Latitude
    units: degrees_north
    axis: Y
Differing data variables:
L   air      (time, lat, lon) float32 241.2 242.5 243.5 ... 296.49 296.19 295.69
R   air      (time, lat, lon) float32 241.2 242.5 243.5 ... 296.49 296.19 295.69
    long_name: 4xDaily Air temperature at sigma level 995
    units: degK
    precision: 2
    GRIB_id: 11
    GRIB_name: TMP
    var_desc: Air temperature
    dataset: NMC Reanalysis
    level_desc: Surface
    statistic: Individual Obs
    parent_stat: Other
    actual_range: [185.16 322.1 ]
```
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