Added app.extend()

Added app.extend(), which provides an easy way to update certain properties of a setting.

lib/application.js

@@ -383,6 +383,50 @@ app.set = function set(setting, val) {
   return this;
 };
 
+/**
+ * Extend `setting` with `val`, or return `setting`'s value.
+ *
+ *    app.set('foo', { 'bar' : 1 })
+ *    app.expand('foo', { 'baz' : 1 });
+ *    app.expand('foo');
+ *    // => { "bar" : 1, "baz" : 1 }
+ *
+ * Mounted servers inherit their parent server's settings.
+ *
+ * @param {String} setting
+ * @param {*} [val]
+ * @return {Server} for chaining
+ * @public
+ */
+
+app.extend = function(setting, val) {
+  if (arguments.length === 1) {
+    // app.get(setting)
+    return this.settings[setting];
+  }
+
+  if (typeof val === 'undefined') {
+    return this;
+  } else if (val === null || typeof val !== 'object') {
+    throw new Error('value needs to be an object');
+  }
+
+  debug('extend "%s" with %o', setting, val);
+
+  var currentSetting = this.settings[setting];
+
+  // no object yet, set value
+  if (currentSetting === null || typeof currentSetting !== 'object') {
+    this.settings[setting] = val;
+    return this;
+  }
+
+  // extend value
+  this.settings[setting] = Object.assign(this.settings[setting], val);
+
+  return this;
+};
+
 /**
  * Return the app's absolute pathname
  * based on the parent(s) that have

test/config.js

@@ -118,6 +118,38 @@ describe('config', function () {
     })
   })
 
+  describe('.extend()', function () {
+    it('should set if value is nonexistent', function () {
+      var app = express();
+      app.extend('foo', { 'bar' : 1 });
+      assert.deepEqual(app.get('foo'), { 'bar' : 1 });
+    })
+
+    it('should extend an object', function () {
+      var app = express();
+      app.extend('foo', { 'bar' : 1 });
+      app.extend('foo', { 'baz' : 1 });
+      assert.deepEqual(app.get('foo'), { 'bar' : 1, 'baz' : 1 });
+    })
+
+    it('should return the app', function () {
+      var app = express();
+      assert.equal(app.extend('foo', { 'bar' : 1 }), app);
+    })
+
+    it('should return the app when undefined', function () {
+      var app = express();
+      assert.equal(app.extend('foo', undefined), app);
+    })
+
+    it('should thorw on bad value', function () {
+      var app = express();
+      (function(){
+        app.extend('foo', null);
+      }).should.throw('value needs to be an object');
+    })
+  })
+
   describe('.enable()', function(){
     it('should set the value to true', function(){
       var app = express();