Fix case when routeNotFound handler is lost when new route is added to the router

[aldas]

Fix case when routeNotFound handler is lost when new route is added to the router.

This happens when new handler we are registering has shorter and partially matching path as router node with routeNotFound handler. In that case when we split existing node into 2 parts we need to assign routeNotFound handler also to split node.

[codecov]

Codecov Report

Merging #2219 (4425059) into master (690e339) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #2219   +/-   ##
=======================================
  Coverage   92.33%   92.34%           
=======================================
  Files          37       37           
  Lines        3118     3122    +4     
=======================================
+ Hits         2879     2883    +4     
  Misses        150      150           
  Partials       89       89           

Impacted Files	Coverage Δ
router.go	96.74% <100.00%> (+0.03%)	⬆️

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