Add conversion of spaces("%20") in request parameters to actual spaces

**Issue by [kelebra](https://github.com/kelebra)**
_Sunday Jun 19, 2016 at 04:05 GMT_
_Originally opened as https://github.com/akka/akka/issues/20802_

---

Hi, guys,

I've been playing around with aka-http and found out that when I have a route like this:

```
get {
path("something" / Rest) { info =>
println(info)
complete(s"You entered: $info")
}
}
```

and send get to `/something/long%20line` the `info` variable is actually equal to `long%20line`. Maybe it would be nice to add at least implicit method to replace `%20` to actual spaces like `info.withSpaces` or it would be make this king of behavior default?

Start by reproducing the request shown in the route example: send /something/long%20line and inspect the Rest value printed by the handler. Trace request-parameter decoding from the route entry point, then verify that the completed value contains an actual space rather than %20.

Written by the indexing model from the issue text.