Fix group.RouteNotFound not working when group has attached middlewares

[aldas]

Fix group.RouteNotFound not working when group has attached middlewares.

Problems is/was that g.Use registers special catch all routes with g.Any and those routes have priority over route registered by g.NotFoundHandler.
Solution is to register these special routes also with NotFoundHandler so if you register custom one - it will override special catch all.

Fixes #2401
For history sake: somewhat relates to #1981 , #2256 , #1728

[aldas]

@lammel please review.

[codecov]

Codecov Report

Base: 92.84% // Head: 92.84% // No change to project coverage 👍

Coverage data is based on head (35a7f4d) compared to base (47844c9).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2411   +/-   ##
=======================================
  Coverage   92.84%   92.84%           
=======================================
  Files          39       39           
  Lines        4503     4503           
=======================================
  Hits         4181     4181           
  Misses        234      234           
  Partials       88       88           

Impacted Files	Coverage Δ
group.go	100.00% <100.00%> (ø)

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[aldas]

@lammel ping

[lammel]

Sorry, late to the party.
For history sake: LGTM