Fix issue in resource model chaining

Fixes #7

src/model/resource.js

@@ -1,5 +1,9 @@
 export default function resource(refEndpoint) {
-    var model = {
+    model = function() {
+        return refEndpoint;
+    }
+
+    var model = Object.assign(model, {
         addRequestInterceptor(interceptor) {
             refEndpoint.requestInterceptors().push(interceptor);
 
@@ -29,9 +33,7 @@ export default function resource(refEndpoint) {
         headers() {
             return refEndpoint.headers();
         },
-    };
+    });
 
-    return Object.assign(function() {
-        return refEndpoint;
-    }, model);
+    return model;
 }

test/src/restful-spec.js

@@ -823,5 +823,24 @@
                 expect(transformedData.title).toBe('Intercepted');
             });
         });
+
+        it('should correctly chain callback in restful object', function() {
+            expect(resource.header('foo', 'bar')).toBe(resource);
+            expect(resource.header('foo', 'bar').prefixUrl('v1')).toBe(resource);
+
+            var articlesCollection = resource.header('foo', 'bar').prefixUrl('v1').all('articles');
+
+            expect(articlesCollection.header('foo1', 'bar1')).toBe(articlesCollection);
+
+            expect(articlesCollection.header('foo2', 'bar2')
+                .addResponseInterceptor(jasmine.createSpy('interceptor'))).toBe(articlesCollection);
+
+            var commentsMember = resource.one('articles', 2).one('comments', 1);
+
+            expect(commentsMember.header('foo3', 'bar3')).toBe(commentsMember);
+
+            expect(commentsMember.header('foo4', 'bar4')
+                .addResponseInterceptor(jasmine.createSpy('interceptor'))).toBe(commentsMember);
+        });
     });
 })();