Don't mutate resource if server responded with no body

If the server provides response with no body to a resource request,
resource should not mutate the resource model in the callback.

src/Resource.js

@@ -97,13 +97,15 @@ ResourceFactory.prototype = {
           data,
           function(status, response, clear) {
             if (status == 200) {
-              if (action.isArray) {
-                value.length = 0;
-                forEach(response, function(item){
-                  value.push(new Resource(item));
-                });
-              } else {
-                copy(response, value);
+              if (response) {
+                if (action.isArray) {
+                  value.length = 0;
+                  forEach(response, function(item){
+                    value.push(new Resource(item));
+                  });
+                } else {
+                  copy(response, value);
+                }
               }
               (callback||noop)(value);
             } else {

test/ResourceSpec.js

@@ -177,6 +177,20 @@ describe("resource", function() {
     expect(callback).wasCalledWith(cc);
   });
 
+  it('should not mutate the resource object if response contains no body', function(){
+    var data = {id:{key:123}, number:'9876'};
+    xhr.expectGET("/CreditCard/123").respond(data);
+    var cc = CreditCard.get({id:123});
+    xhr.flush();
+    expect(cc instanceof CreditCard).toBeTruthy();
+    var idBefore = cc.id;
+
+    xhr.expectPOST("/CreditCard/123", data).respond('');
+    cc.$save();
+    xhr.flush();
+    expect(idBefore).toEqual(cc.id);
+  });
+
   it('should bind default parameters', function(){
     xhr.expectGET('/CreditCard/123.visa?minimum=0.05').respond({id:123});
     var Visa = CreditCard.bind({verb:'.visa', minimum:0.05});