Fix transliteration bug

[Adriandmen]

Description

As Kevin Cruijssen pointed out, transliteration does not work properly when one of the parameters is an integer. This is now fixed by replacing is_bitstring with is_single? (which checks whether an element is a string or number) and always casts to a string if this is the case.

Tests are now also added to make sure that this does not happen again.

[codecov]

Codecov Report

Merging #123 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #123      +/-   ##
==========================================
- Coverage   95.82%   95.81%   -0.01%     
==========================================
  Files          17       17              
  Lines        1723     1722       -1     
==========================================
- Hits         1651     1650       -1     
  Misses         72       72

Impacted Files	Coverage Δ
lib/commands/str_commands.ex	91.96% <100%> (-0.08%)	⬇️

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