unique merge array customisation fix

[trombka]

This fixes #161

[codecov-io]

Codecov Report

Merging #162 (54271a5) into develop (57ec295) will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #162      +/-   ##
===========================================
- Coverage    98.50%   98.47%   -0.03%     
===========================================
  Files            6        6              
  Lines          200      197       -3     
  Branches        56       55       -1     
===========================================
- Hits           197      194       -3     
  Misses           3        3              

Impacted Files	Coverage Δ
src/unique.ts	100.00% <100.00%> (ø)

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[bebraw]

The fix is included in 5.4.1. Thanks a lot for contributing!