add more test for `@when`

[inkydragon]

• fix broken test & add new test
• add some comments
• fix typo in docstring
• fix BoundsError in SubString(s, 1, 20) when length(s) < 20

[codecov[bot]]

Codecov Report

Merging #60 into master will increase coverage by 0.41%. The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #60      +/-   ##
==========================================
+ Coverage   90.05%   90.46%   +0.41%     
==========================================
  Files          19       19              
  Lines         553      556       +3     
==========================================
+ Hits          498      503       +5     
+ Misses         55       53       -2

Impacted Files	Coverage Δ
src/StandardPatterns/WhenCases.jl	100% <100%> (+4.25%)	:arrow_up:

---

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 ed0c499...fbd69e0. Read the comment docs.

[thautwarm]

Really cautious..