`restify.clj`: list-all-in-collection route now supports query string pa...

* specifying a `format` will change output format (supported: `csv` or `json`; defaults to `json` if omitted)
* specifying a `filename` will force a `Content-Disposition: attachment` header
Also I tend to prefer when possible to drop conditional logic to the call site rather than living in the helper functions. So for here download would always expect a filename argument and we could just be obvious an bind the resp to a name and 

```clojure
(if filename
  (downloadable resp filename)
  resp)
```
or if we wanted to keep the threading and not bind as much in the toplevel of the fn
```
(-> etc
   (#(if-let [filename (some-> req :query-params :filename)]
               (downloadable % filename)
               %)))
```
So that lets us keep the "type" of the download helper a bit simpler.