fix hdi failure with boolean array

[aloctavodia]

Fix #1178

[codecov]

Codecov Report

Merging #1444 (5d8523c) into master (5d2a4e1) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1444   +/-   ##
=======================================
  Coverage   91.94%   91.94%           
=======================================
  Files         105      105           
  Lines       11216    11216           
=======================================
  Hits        10312    10312           
  Misses        904      904           

Impacted Files	Coverage Δ
arviz/stats/stats.py	96.39% <100.00%> (ø)

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