Resolve "FutureWarning: xarray.merge will change"

[btschwertfeger]

Setting the current default values explicitly ensures future releases of xarray won't break cmethods.

Closes #178

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.82%. Comparing base (2c5814f) to head (fd17d9b).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #179   +/-   ##
=======================================
  Coverage   95.82%   95.82%           
=======================================
  Files           7        7           
  Lines         335      335           
=======================================
  Hits          321      321           
  Misses         14       14           

Flag	Coverage Δ
unittests	95.82% <100.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
cmethods/core.py	90.27% <100.00%> (ø)

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