Improve find code quality, fix find-by-tag casing bug

[kokrui]

Closes #229
Closes #228

[codecov]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (33db2cf) 82.72% compared to head (9baa582) 82.75%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #230      +/-   ##
============================================
+ Coverage     82.72%   82.75%   +0.02%     
- Complexity      763      769       +6     
============================================
  Files           100      100              
  Lines          2356     2383      +27     
  Branches        262      264       +2     
============================================
+ Hits           1949     1972      +23     
- Misses          337      340       +3     
- Partials         70       71       +1     

Files	Coverage Δ
...in/java/seedu/address/commons/util/StringUtil.java	96.66% <100.00%> (+0.83%)	⬆️
...java/seedu/address/logic/commands/FindCommand.java	100.00% <ø> (ø)
...edu/address/logic/parser/FindExpressionParser.java	94.02% <87.17%> (-1.81%)	⬇️

... and 2 files with indirect coverage changes

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[lordidiot]

LGTM! Appreciate the detailed javadocs and the additional comments for future developers to better understand the codebase.