Revert back to v4 behavior for group registering implicit 404 handler…

[aldas]

FIX: #2950

Revert back to v4 behavior for group registering implicit 404 handlers. This will fix: CORS middleware doesnt automatically handle OPTIONS routes for groups anymore since upgrade to v5.

This is a breaking change in v5. It reverts back to v4 behavior.

[codecov]

Codecov Report

❌ Patch coverage is 90.47619% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.20%. Comparing base (01b45be) to head (662f4d0).
⚠️ Report is 8 commits behind head on master.

Files with missing lines	Patch %	Lines
router.go	71.42%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2996      +/-   ##
==========================================
+ Coverage   93.17%   93.20%   +0.02%     
==========================================
  Files          43       43              
  Lines        4501     4517      +16     
==========================================
+ Hits         4194     4210      +16     
- Misses        189      190       +1     
+ Partials      118      117       -1     

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