Don't depend on route-params implementing IFn when rendering routes.

Fixes a bug when passed a record as route-params as it doesn't implement
```IFn``` like normal maps do.

This allows you to write code like:

```clojure
(defrecord Foo [id]]

(defroute foo-route "/foo/:id" [id]
  ...)

(let [some-foo (->Foo 42)]
  [:a {:href (foo-route some-foo)}])
```

I know that I could extend ```Foo``` with ```IRenderRoute``` in this (simplified) example, but there are some cases in my application where that wouldn't be an optimal solution. In addition, secretary shouldn't depend on the map implementing ```IFn``` in the same way ```get``` would behave.