Add test for keyed Int types

[hodovani]

Add test for keyd Int, Int8, Int16, Int32, Int64, UInt, UInt8, UInt16, UInt32, UInt64 types.

[codecov]

Codecov Report

Merging #58 into master will increase coverage by 2.09%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master      #58      +/-   ##
==========================================
+ Coverage   66.38%   68.48%   +2.09%     
==========================================
  Files          30       30              
  Lines        1431     1431              
==========================================
+ Hits          950      980      +30     
+ Misses        481      451      -30

Impacted Files	Coverage Δ
...s/XMLCoder/Decoder/XMLKeyedDecodingContainer.swift	78.72% <0%> (+11.34%)	⬆️
...s/XMLCoder/Encoder/XMLKeyedEncodingContainer.swift	61.44% <0%> (+16.86%)	⬆️

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