:refer doesn't allow to override existing vars

[mk]

version
git dep at 4c920c0.

platform
OS X
problem

Since upgrading from 0.2.1 to 4c920c0 I noticed the following change: in 0.2.1 :refer :all would overwrite + from clojure.core, in the latest master it would not.

I've bisected and found f8f4268 as the commit that introduced the change in behavior.

repro
Evaluating

(ns test.repro)
(def + str)
(require '[test.repro :refer [+]])

followed by

(+ 1 2)

Should return "12", not 3.

[borkdude]

Small repro:

(sci/eval-string "(defn join []) (refer 'clojure.string) (ns-name (:ns (meta #'join)))")

This should return clojure.string instead of user.

[borkdude]

@mk Fixed with 55b9f55