fix bug with skin caching that cached capes as non-existent

when using cache.save_hash, an `undefined` skin or cape hash means "do not touch"
while `null` means "overwrite as non-existent"

this was sent to redis after calling /capes/Notch for the first time:

  "hmset" "notch" "c" "3f688e0e699b3d9fe448b5bb50a3a288f9c589762b3dae8308842122dcb81" "t" "1429651244222"
  "hmset" "notch" "s" "a116e69a845e227f7ca1fdde8c357c8c821ebd4ba619382ea4a1f87d4ae94" "c" "" "t" "1429651244235"

as you can see, the first request stores the c(ape) but does not touch the s(kin), whereas the second request
sets the s(kin) and replaces the c(ape) value

lib/helpers.js

@@ -177,7 +177,7 @@ function store_images(rid, userId, cache_details, type, callback) {
             // an error occured, not caching. we can try in 60 seconds
             callback_for(userId, "skin", store_err, null);
           } else {
-            cache.save_hash(rid, userId, skin_hash, null, function(cache_err) {
+            cache.save_hash(rid, userId, skin_hash, undefined, function(cache_err) {
               callback_for(userId, "skin", (store_err || cache_err), skin_hash);
             });
           }