Cast array to slice. Fixes #719

[kamphaus]

Fixes #719

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This change is 

[codecov]

Codecov Report

Merging #720 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #720      +/-   ##
==========================================
+ Coverage   82.03%   82.04%   +<.01%     
==========================================
  Files         164      164              
  Lines        9057     9062       +5     
==========================================
+ Hits         7430     7435       +5     
  Misses       1223     1223              
  Partials      404      404

Impacted Files	Coverage Δ
types/cast.go	69.89% <100%> (+0.52%)	⬆️

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[elliotchance]

Reviewed 2 of 2 files at r1.
Review status: all files reviewed at latest revision, all discussions resolved.

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