Extracted URL coding logic into URLBox with corresponding unit tests

[regexident]

No description provided.

[codecov]

Codecov Report

Merging #30 into master will increase coverage by 0.75%.
The diff coverage is 81.25%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   50.55%   51.31%   +0.75%     
==========================================
  Files          28       29       +1     
  Lines        1351     1374      +23     
==========================================
+ Hits          683      705      +22     
- Misses        668      669       +1

Impacted Files	Coverage Δ
Sources/XMLCoder/Encoder/XMLEncoder.swift	48.21% <100%> (+1.51%)	⬆️
Sources/XMLCoder/Decoder/XMLDecoder.swift	39.69% <66.66%> (+1.77%)	⬆️
Sources/XMLCoder/Box/URLBox.swift	85.71% <85.71%> (ø)

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[regexident]

Rebased. Ready to merge.