Support for array values in query-string and body

One of my services expect array values in request parameters (multiple parameters with the same name), something like this:
```clojure
(let [req (-> (request :post "/search")
                 (body {:body "kontera"
                           :age true
                           :tag ["one" "two"]
                           :max-docs "10"}))
       res (web-app req)
       body (parse-string (:body res) true)]
  ;;....
)
```
I added support for it in ```ring.mock.request/encode-params``` and added few tests.