Fix leaky slash when normalizing

[ericproulx]

Fix leaky slash handling in router

Summary

This PR addresses an issue with leaky slash handling in the router. While profiling grape-on-rack locally, I found multiple / that were retained for nothing.

Retained String Report
-----------------------------------
        28  "/"
        27  grape-2.4.0/lib/grape/router.rb:18
         1  grape-2.4.0/lib/grape/router.rb:22

[grape-bot]

	1 Warning
⚠️	There're library changes, but not tests. That's OK as long as you're refactoring existing code.

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