Zonal_crosstab 3D: Ensure content of input param values is preserved

[thuydotm]

Fixes #751

[codecov-commenter]

Codecov Report

Base: 79.87% // Head: 79.88% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (363457d) compared to base (f6c5164).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #754   +/-   ##
=======================================
  Coverage   79.87%   79.88%           
=======================================
  Files          19       19           
  Lines        4169     4170    +1     
=======================================
+ Hits         3330     3331    +1     
  Misses        839      839           

Impacted Files	Coverage Δ
xrspatial/zonal.py	85.47% <100.00%> (+0.02%)	⬆️

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[brendancol]

@thuydotm

• Add test case to test_zonal to make sure input values remain unmodified