construct full url to pass to MockRequest#env_for

I came accross issue #5 while testing reel with rack urlmap. It's not working because `SERVER_NAME` and `SERVER_PORT` do not get set correctly (see https://github.com/rack/rack/blob/master/lib/rack/mock.rb#L88).

I modified this so that a more complete url is passed to `env_for`. I don't know if it's the right way to go.

* I'm still missing the scheme (just picking http for now)
* Maybe `Reel::Request#url` should return a full url?

What do you think?