Add request url to corresponding response object

Not sure if this is something that people want. Currently I have the need to access the URL of a response object. I basically just wrapped a custom struct on top of Response and added a field `url`. But this means that I need to delegate a lot of the functions of `Response`.

As 2.0 is being worked on, would you guys consider such a change? I can implement it and open a PR if this feature would get merged into.