Always ravel array in unique_array

[Hoxbro]

Fixes #5970

[codecov-commenter]

Codecov Report

Merging #5969 (884146e) into main (3adb998) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #5969   +/-   ##
=======================================
  Coverage   88.58%   88.58%           
=======================================
  Files         315      315           
  Lines       65500    65504    +4     
=======================================
+ Hits        58020    58024    +4     
  Misses       7480     7480           

Flag	Coverage Δ
ui-tests	23.15% <20.00%> (+<0.01%)	⬆️

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Files	Coverage Δ
holoviews/core/util.py	85.45% <100.00%> (ø)
holoviews/tests/core/test_utils.py	99.22% <100.00%> (+<0.01%)	⬆️

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