Fix overwriting existing routes with new route on / path

[mthenw]

There was a bug allowing causing overwriting existing route with new route on root / level.

Todos:

• Write tests
• Write documentation
• Fix linting errors
• Make sure code coverage hasn't dropped
• Provide verification commands / resources
• Enable "Allow edits from maintainers" for this PR
• Update the messages below

Is this ready for review?: YES
Is it a breaking change?: YES

[codecov]

Codecov Report

Merging #472 into master will decrease coverage by 0.06%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #472      +/-   ##
==========================================
- Coverage   70.78%   70.71%   -0.07%     
==========================================
  Files          35       35              
  Lines        2163     2165       +2     
==========================================
  Hits         1531     1531              
- Misses        570      571       +1     
- Partials       62       63       +1

Impacted Files	Coverage Δ
libkv/subscription.go	88.75% <100%> (ø)	⬆️
internal/pathtree/tree.go	95.23% <100%> (-1.85%)	⬇️

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