Fix popstate handler sending wrong parameters to scrollBehavior

[mutoo]

I found that the scrollBehavior callback received the same route object as to and from parameters when users click the browser's back/forward button.

Then I followed the call stacks and found that there was a one line bug at history/html5.js

    window.addEventListener('popstate', e => {
      this.transitionTo(getLocation(this.base), route => {
        if (expectScroll) {
          handleScroll(router, route, this.current, true)
        }
      })
    })

Actually the this.current is changed before the callback be called.

Since it's a bit hard to provide a history-mode demo in codepen/jsfiddle, and the bug is so obvious.
There is any demo be provided. Sorry about that.

version: vue-router 2.5.3

[posva]

ping @yyx990803