issues/436: Fix a number of CORS issues

[komuw]

If allowedOrigins is nil, domain and its www variant are used
Validate allowed origins, methods, headers, etc.

• Fixes: ong/cors: Address a few issues #436
  Hat tip:
• https://jub0bs.com/posts/2023-02-08-fearless-cors/ : This is a really great blogpost on all things CORS.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 86.25000% with 22 lines in your changes missing coverage. Please review.

Project coverage is 70.46%. Comparing base (94b4f3f) to head (c47532e).
Report is 8 commits behind head on main.

Files	Patch %	Lines
middleware/cors.go	86.16%	13 Missing and 9 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #442      +/-   ##
==========================================
+ Coverage   69.75%   70.46%   +0.70%     
==========================================
  Files          52       54       +2     
  Lines        6137     6352     +215     
==========================================
+ Hits         4281     4476     +195     
- Misses       1541     1551      +10     
- Partials      315      325      +10     

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