Fix bug aborting popstate

I hit the bug reported in this issue #5210 a few days ago. 

tl;dr
When you hit the back button and the transition is aborted, the url has already been updated is not rollbacked.

I've managed to fix it, but I have no clue about how to test it, because I would involve set some routes and play with the history. Seems almost an acceptance test to me.
@cibernox Yep. You're right. How about other transitions though? Wouldn't this code path be hit for other aborted transitions as well, including forward button and all other in-app transitions? In a common transition that pushes, this would push again? My apologizes if I'm totally misunderstanding. If this is the case, we perhaps need some sort of transaction-style saving of what is currently underway mid-transition so we know exactly which way to rollback.