Find more elegant way to call baseURL

Up to this point if we set `baseUrl = https://www.server.com/api` and we have an API which needs to call the baseURL, we need to write a func like this:

```swift
func getRoot() -> Single<Root> {
    return request(url: absoluteURL(""))
}
```
`absoluteURL("")` is really ugly, we should find a more elegant way