🐛 bug: fix mounting when mount prefix is /

[efectn]

Description

Fix mounting scenarios like

app := New()
dynamic := New()
apiserver := New()

apiroutes := apiserver.Group("/v1")
apiroutes.Get("/home", func(c *Ctx) error {
	return c.SendString("home")
})

dynamic.Mount("/api", apiserver)
app.Mount("/", dynamic)

Type of change

• Bug fix (non-breaking change which fixes an issue)

Checklist:

• I have performed a self-review of my own code
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

Commit formatting:

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