CLJ-940: check for sequential collection after :only/:refer

Throws more informative error in the following cases:

    (ns (:require [foo :refer bar]))

    (ns (:use [foo :only bar]))

    (refer 'foo :only 'bar)

Signed-off-by: Stuart Halloway <[email protected]>

Author: Stuart Sierra

src/clj/clojure/core.clj

@@ -3800,6 +3800,8 @@
           to-do (if (= :all (:refer fs))
                   (keys nspublics)
                   (or (:refer fs) (:only fs) (keys nspublics)))]
+      (when-not (instance? clojure.lang.Sequential to-do)
+        (throw (new Exception ":only/:refer value must be a sequential collection of symbols")))
       (doseq [sym to-do]
         (when-not (exclude sym)
           (let [v (nspublics sym)]