convert non-string default params to strings in the router.

Closes #9435.

Since Rails 4.0.0.beta1 Rails raises an exception when non-string default params
are provided in the router (eg. `root to: 'main#posts', page: 1`).

I see sevaral options to fix this:

1. issue a deprecation warning when non-string defaults are used.
2. convert all default params to strings (implemented by this patch).
3. get the router to work with non-string params.

All three options can be implemented with minimal effort.