Increase test coverage

[hodovani]

Add tests to increase code coverage in Decoder files

[codecov]

Codecov Report

Merging #56 into master will increase coverage by 10.2%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   56.18%   66.38%   +10.2%     
==========================================
  Files          30       30              
  Lines        1431     1431              
==========================================
+ Hits          804      950     +146     
+ Misses        627      481     -146

Impacted Files	Coverage Δ
...s/XMLCoder/Encoder/XMLKeyedEncodingContainer.swift	44.57% <0%> (+2.4%)	⬆️
Sources/XMLCoder/Decoder/XMLDecoder.swift	53.14% <0%> (+14.49%)	⬆️
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	67.37% <0%> (+21.98%)	⬆️
Sources/XMLCoder/Encoder/XMLEncoder.swift	74.88% <0%> (+27.31%)	⬆️
...urces/XMLCoder/Encoder/XMLReferencingEncoder.swift	56.75% <0%> (+56.75%)	⬆️

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