Fix trailing slash on root

Related to https://github.com/chaplinjs/chaplin/issues/838

I don't know how to add a test without modifing the code as I need to test the internal variable `url` of `History.navigate`

If you want, it can extract the following code in a function so that I would be able to test it.
```
url = @root + fragment
# Don't include a trailing slash on the root.
if fragment.length is 0 and url isnt @root
  url = url.slice 0, -1
```