alter :path-info as well as :uri

Hey all!

I'm just spinning up a new compojure app and dieter seems like the right lib for me to handle less compilation. However, I noticed that when deploying to immutant dieter wasn't playing nice. So after lots of digging I discovered that the :path-info key in a request is actually preferred over :uri by ring when serving static content. So I wrote a quick patch.

As an aside, issue #48 has also been a problem for me. If you guys are accepting contributions I might be tempted to try a fix for that as well. 

I threw some more detail in the commit message, see below.

When changing the path of a request, be sure to change :path-info as
well as :uri.

Ring actually looks at :path-info *first*. See
https://github.com/ring-clojure/ring/blob/1.2/ring-core/src/ring/middleware/file.clj#L21
and
https://github.com/ring-clojure/ring/blob/1.2/ring-core/src/ring/util/request.clj#L32-L36

So, when we're running on a server that supplies :path-info, we have to
be sure it is correct, or else we'll end up either 404'ing (bad) or
serving the unaltered content (possibly worse).