Update call to xarray _validate_attrs

[iainrussell]

Fixes #400.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes missing coverage. Please review.

Project coverage is 41.61%. Comparing base (454510e) to head (51425cf).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
cfgrib/xarray_to_grib.py	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #401      +/-   ##
==========================================
+ Coverage   41.40%   41.61%   +0.21%     
==========================================
  Files          26       26              
  Lines        2198     2201       +3     
  Branches      285      285              
==========================================
+ Hits          910      916       +6     
+ Misses       1288     1285       -3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.