setServerUrl produces URLs with double slashes when URL already ends with one

works:
```
val url = "https://some.url"
setServer(url) // https://some.url/track
```

broken:
```
val url = "https://some.url/"
setServer(url) // https://some.url//track
```

This makes writing rewrite rules painful. Mixpanel should not just blindly append Strings but produces well-formed URLs.