Improve code coverage of box types

[regexident]

… and remove unused internal methods to reduce the maintenance burden.

[codecov]

Codecov Report

Merging #42 into master will increase coverage by 2.79%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master     #42      +/-   ##
=========================================
+ Coverage   51.41%   54.2%   +2.79%     
=========================================
  Files          29      29              
  Lines        1412    1402      -10     
=========================================
+ Hits          726     760      +34     
+ Misses        686     642      -44

Impacted Files	Coverage Δ
Sources/XMLCoder/Box/KeyedBox.swift	100% <100%> (+22.72%)	⬆️
Sources/XMLCoder/Box/UnkeyedBox.swift	100% <100%> (+42.85%)	⬆️
Sources/XMLCoder/Box/DateBox.swift	95.74% <0%> (+4.25%)	⬆️
Sources/XMLCoder/Box/FloatBox.swift	100% <0%> (+10%)	⬆️
Sources/XMLCoder/Box/DataBox.swift	100% <0%> (+10.52%)	⬆️
Sources/XMLCoder/Box/BoolBox.swift	100% <0%> (+13.33%)	⬆️
Sources/XMLCoder/Box/IntBox.swift	100% <0%> (+14.28%)	⬆️
Sources/XMLCoder/Box/URLBox.swift	100% <0%> (+14.28%)	⬆️
... and 3 more

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 3d22e01...267c7ed. Read the comment docs.

[regexident]

Ready to merge.

[MaxDesiatov]

Thanks, a bit unclear whether a test case property box should be a variable instead of a constant.

[regexident]

Ugh, turns out renaming a branch backing a PR is not best idea I ever had. 😅🙈