sort fields properly

[alovak]

This PR fixes the issue with converting subfield indexes (map keys) into int to sort them as ints and then convert them back to string so we can use them as keys for the map.

The issue happened when key 01 (string) converted into 1 (int) and then into 1 (string). Code panic with invalid memory address or nil pointer dereference.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: +0.01% 🎉

Comparison is base (36606f8) 73.49% compared to head (78ade2f) 73.50%.

❗ Current head 78ade2f differs from pull request most recent head 545fc16. Consider uploading reports for the commit 545fc16 to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #259      +/-   ##
==========================================
+ Coverage   73.49%   73.50%   +0.01%     
==========================================
  Files          43       43              
  Lines        2290     2291       +1     
==========================================
+ Hits         1683     1684       +1     
- Misses        375      377       +2     
+ Partials      232      230       -2     

see 1 file with indirect coverage changes

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