fix(select): make a collection array copy to avoid change by ref

[ghiscoding]

• when using "addBlankEntry" or "addCustomFirstEntry", the SelectFilter was affecting SelectEditor (and vice versa) because the collection was used by ref, so to fix this we can just do a copy of the collection before dealing with it

[codecov]

Codecov Report

Merging #595 into master will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master      #595   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files          149       149           
  Lines         9939      9945    +6     
  Branches      3513      3514    +1     
=========================================
+ Hits          9939      9945    +6     

Impacted Files	Coverage Δ
.../modules/angular-slickgrid/editors/selectEditor.ts	100.00% <100.00%> (ø)
.../modules/angular-slickgrid/filters/selectFilter.ts	100.00% <100.00%> (ø)

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