Supports use as a function

[Jie]

Purpose

fun-map has replaced clojure map in most situations, except for one case where it cannot be used as a function, such as the expected call

(def m (fun-map {:a 1}))
(m :a)   ;; => 1
(m :b 2) ;; => 2

but actually get an error: class robertluo.fun_map.core.DelegatedMap cannot be cast to class clojure.lang.IFn.

[Luo Tian]

It is a good addition. Could you send me a PR for this?