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
391168098,MDU6SXNzdWUzOTExNjgwOTg=,2606,Can't decode cf_variable,1411854,closed,0,,,1,2018-12-14T16:00:32Z,2018-12-14T16:28:29Z,2018-12-14T16:28:29Z,NONE,,,,"Hi,

#### Problem description

I'm trying to decode a CF variable that contains required keys/values in encoding, but the values are not decoded when calling `xarray.conventions.decode_cf_variable``. I'm using dask arrays because the feature I'm developing is supposed to be scaled with dask.

#### Code Sample, a copy-pastable example if possible

```python
import dask
import xarray as xr
from numpy import dtype
from collections import OrderedDict

name='lsm'
encoded_var = xr.Variable(dims=('latitude','longitude'),
                          data=dask.array.from_array([[-11608,   9419,  27983,  32767,   1306],
                               [ 24657,  27490,  12354,  13565, -31678],
                               [ 23286,  19197,   -425, -17866,  -2387],
                               [  8284,  22922,  25428,  28584,  29309],
                               [ 21439,  26654,  15559,  -2752, -32766]],
                                                    chunks=(5,5)),
                          attrs=OrderedDict([('long_name', 'Land-sea mask'),
                             ('standard_name', 'land_binary_mask'),
                             ('units', '(0 - 1)')]),
                          encoding={'chunks': (5, 5),
                             '_FillValue': -32767,
                             'dtype': dtype('int16'),
                             'scale_factor': 1.0576998063464478e-07,
                             'add_offset': 0.9965342350445446}
                         )
encoded_var[:].compute()
```
```
<xarray.Variable (latitude: 5, longitude: 5)>
array([[-11608,   9419,  27983,  32767,   1306],
       [ 24657,  27490,  12354,  13565, -31678],
       [ 23286,  19197,   -425, -17866,  -2387],
       [  8284,  22922,  25428,  28584,  29309],
       [ 21439,  26654,  15559,  -2752, -32766]])
Attributes:
    long_name:      Land-sea mask
    standard_name:  land_binary_mask
    units:          (0 - 1)
```

Then when trying to decode, I obtain the same values:

```python
decoded_var = xr.conventions.decode_cf_variable(name, encoded_var)
decoded_var[:].compute()
```
```
<xarray.Variable (latitude: 5, longitude: 5)>
array([[-11608,   9419,  27983,  32767,   1306],
       [ 24657,  27490,  12354,  13565, -31678],
       [ 23286,  19197,   -425, -17866,  -2387],
       [  8284,  22922,  25428,  28584,  29309],
       [ 21439,  26654,  15559,  -2752, -32766]])
Attributes:
    long_name:      Land-sea mask
    standard_name:  land_binary_mask
    units:          (0 - 1)
```

#### Expected Output
```
<xarray.Variable (latitude: 5, longitude: 5)>
array([[0.99530005, 0.99752796, 0.99949646, 1.        , 0.99667346],
       [0.9991455 , 0.9994354 , 0.9978484 , 0.99797046, 0.993179  ],
       [0.99899286, 0.9985656 , 0.99649036, 0.9946439 , 0.9962767 ],
       [0.9974059 , 0.99896234, 0.9992218 , 0.9995575 , 0.9996338 ],
       [0.9987945 , 0.99935913, 0.9981841 , 0.99624616, 0.99307215]])
Attributes:
    long_name:      Land-sea mask
    standard_name:  land_binary_mask
    units:          (0 - 1)

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.7.final.0
python-bits: 64
OS: Linux
OS-release: 4.19.6-1-MANJARO
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_GB.UTF-8
LOCALE: en_GB.UTF-8

xarray: 0.11.0
pandas: 0.23.4
numpy: 1.14.3
scipy: 1.1.0
netCDF4: 1.4.2
h5netcdf: None
h5py: None
Nio: None
zarr: 2.2.0
cftime: 1.0.2.1
PseudonetCDF: None
rasterio: None
iris: None
bottleneck: None
cyordereddict: None
dask: 0.20.2
distributed: 1.24.2
matplotlib: 2.2.3
cartopy: 0.16.0
seaborn: None
setuptools: 40.6.0
pip: 18.1
conda: None
pytest: None
IPython: 7.1.1
sphinx: None

</details>
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