Add beforechange event type and provide key with change events

If you know ahead of time that you only want to listen for changes for a specific property, the foo:change type events can be useful.  If you want to listen for changes on all properties, the change event becomes more useful if it provides information on what changed.  And the beforechange event allows listeners to access values before they change.

test/spec/ol/object.test.js

@@ -121,7 +121,11 @@ describe('ol.Object', function() {
 
     it('dispatches generic change events to bound objects', function() {
       o.notify('k');
-      expect(listener2).to.be.called();
+      expect(listener2.calledOnce).to.be(true);
+      var args = listener2.firstCall.args;
+      expect(args).to.have.length(1);
+      var event = args[0];
+      expect(event.key).to.be('k');
     });
 
     it('dispatches events to bound objects', function() {
@@ -157,7 +161,25 @@ describe('ol.Object', function() {
 
     it('dispatches generic change events to object', function() {
       o.set('k', 1);
-      expect(listener2).to.be.called();
+      expect(listener2.calledOnce).to.be(true);
+      var args = listener2.firstCall.args;
+      expect(args).to.have.length(1);
+      var event = args[0];
+      expect(event.key).to.be('k');
+    });
+
+    it('dispatches beforechange events to object', function() {
+      o.set('k', 1);
+
+      var oldValue;
+      var beforeListener = sinon.spy(function(event) {
+        oldValue = o2.get(event.key);
+      });
+      o.on('beforechange', beforeListener);
+
+      o.set('k', 2);
+      expect(beforeListener.calledOnce).to.be(true);
+      expect(oldValue).to.be(1);
     });
 
     it('dispatches events to bound object', function() {
@@ -175,8 +197,32 @@ describe('ol.Object', function() {
 
     it('dispatches generic change events to object bound to', function() {
       o2.set('k', 2);
-      expect(listener2).to.be.called();
+      expect(listener2.calledOnce).to.be(true);
+      var args = listener2.firstCall.args;
+      expect(args).to.have.length(1);
+      var event = args[0];
+      expect(event.key).to.be('k');
     });
+
+    it('dispatches beforechange before changing bound objects', function() {
+      o2.set('k', 1);
+
+      var oldValue;
+      var beforeListener = sinon.spy(function(event) {
+        oldValue = o2.get(event.key);
+      });
+      o.on('beforechange', beforeListener);
+
+      o2.set('k', 2);
+      expect(beforeListener.calledOnce).to.be(true);
+      var args = listener2.firstCall.args;
+      expect(args).to.have.length(1);
+      var event = args[0];
+      expect(event.key).to.be('k');
+
+      expect(oldValue).to.be(1);
+    });
+
   });
 
   describe('bind', function() {