Fix popstate safari workaround

We also need to reset the "initial href to ignore" whenever storing a new state.
Otherwise you get this behaviour:
1. Visit landing page A
2. Navigate to other page B (`cm.router.route()`)
3. Click "back" in browser. -> Should go to A but does not - because the "initial href" is still set

@vogdb please review

I'm gonna merge and release this in the meantime.