Add tests to increase test coverage

[hodovani]

• Add test for Decode .keyFormatted

[codecov]

Codecov Report

Merging #63 into master will increase coverage by 2.63%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #63      +/-   ##
=========================================
+ Coverage   70.76%   73.4%   +2.63%     
=========================================
  Files          30      30              
  Lines        1440    1440              
=========================================
+ Hits         1019    1057      +38     
+ Misses        421     383      -38

Impacted Files	Coverage Δ
Sources/XMLCoder/Decoder/XMLDecoder.swift	70.04% <0%> (+16.9%)	⬆️
...rces/XMLCoder/Decoder/DecodingErrorExtension.swift	100% <0%> (+100%)	⬆️

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