react-router: False positives for active Links

The current implementation of pathnameIsActive gives false positives for what I’d expect to be a fairly common case: numbers in path parameters.

Just hit this in v1.0.0-beta2 in an app which is using the index of some items in an array as a parameter. Once you start hitting double-digit numbers, you get false positives from single-digit numbers.

In this example, the URL is /1/10 and links for both /1/1 and /1/10 are marked active.