Groupby reduce with string grouping keys fix

[krynju]

a = DTable((a=rand(["abc", "bcd", "efg"], 100), b=rand(100)), 11)
r = fetch(reduce((x, y) -> x + y, DTables.groupby(a, :a), cols=[:b], init=0))

it would return r.a = ['a', 'b', 'e']

[codecov-commenter]

Codecov Report

Base: 95.85% // Head: 95.99% // Increases project coverage by +0.13% 🎉

Coverage data is based on head (c5e5ac8) compared to base (39ef120).
Patch coverage: 100.00% of modified lines in pull request are covered.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main      #24      +/-   ##
==========================================
+ Coverage   95.85%   95.99%   +0.13%     
==========================================
  Files          10       11       +1     
  Lines         820      823       +3     
==========================================
+ Hits          786      790       +4     
+ Misses         34       33       -1     

Impacted Files	Coverage Δ
src/operations/operations.jl	98.50% <100.00%> (+0.04%)	⬆️
src/DTables.jl	100.00% <0.00%> (ø)
src/table/dtable.jl	92.85% <0.00%> (+0.79%)	⬆️

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