fix: field indexes getting overridden when appending

[doug-martin]

Fix for #290

When creating the columnMap with multiple levels of embedded structs the append was mutating the original fieldIndex slice causing the fieldIndex for each column to be incorrect.

[codecov]

Codecov Report

Merging #291 (27530d6) into master (e7dd58b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #291   +/-   ##
=======================================
  Coverage   96.80%   96.80%           
=======================================
  Files          61       61           
  Lines        3350     3353    +3     
=======================================
+ Hits         3243     3246    +3     
  Misses         91       91           
  Partials       16       16           

Flag	Coverage Δ
unittests	96.80% <100.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
internal/util/column_map.go	100.00% <100.00%> (ø)

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