[#120] Update freezable? to cover nil

src/taoensso/nippy/utils.clj

@@ -93,6 +93,9 @@
   test for pre/post serialization value equality (there's no good general
   way of doing so)."
 
+  ;; TODO Not happy with this approach in general, could do with a refactor.
+  ;; Maybe return true/false/nil (nil => maybe)?
+
   ([x] (freezable? x nil))
   ([x {:keys [allow-clojure-reader? allow-java-serializable?]}]
    (if (is-coll? x)
@@ -102,6 +105,7 @@
        (is? x java.lang.String)
        (is? x java.lang.Long)
        (is? x java.lang.Double)
+       (nil? x)
 
        (is? x clojure.lang.BigInt)
        (is? x clojure.lang.Ratio)