modify_url fix for paths without trailing slash

Before:

```R
modify_url('http://google.com?a=1', query = list(b = 2))
# "http://google.com?a=1/?b=2"
```

After:

```R
modify_url('http://google.com?a=1', query = list(b = 2))
# "http://google.com/?a=1&b=2"
```