[Fix] calling navigate with number throws

[baptooo]

Hello, first thank you for the wonderful job I am using reach router for more than a year now and I am super happy with it ! 👏

I am raising this pull request as I faced an issue using navigate function with a number

The issue

When calling the function navigate with a number as it is mentioned in the documentation

You can pass a number to go to a previously visited route.
navigate(-1)

It results into a crash because the function startsWith requires a string and is called with to argument

The fix

Escaping the case where to argument is a number