Fix memory adapter updateAttributes issue.

Callback with an error if an id was not provided or if a model was not
found by that id.

lib/adapters/memory.js

@@ -229,9 +229,26 @@ Memory.prototype.count = function count(model, callback, where) {
 };
 
 Memory.prototype.updateAttributes = function updateAttributes(model, id, data, cb) {
+    if(!id) {
+      var err = new Error('You must provide an id when updating attributes!');
+      if(cb) {
+        return cb(err);
+      } else {
+        throw err;
+      }
+    }
+  
     data.id = id;
-    var base = JSON.parse(this.cache[model][id]);
+    
+    var cachedModels = this.cache[model];
+    var modelAsString = cachedModels && this.cache[model][id];
+    var modelData = modelAsString && JSON.parse(modelAsString);
+    
+    if(modelData) {
       this.save(model, merge(base, data), cb);
+    } else {
+      cb(new Error('Could not update attributes. Object with id ' + id + ' does not exist!'));
+    }
 };
 
 Memory.prototype.transaction = function () {