fix($compile): don't trigger $observer if initial value is undefined

src/ng/compile.js

@@ -1188,7 +1188,7 @@ function $CompileProvider($provide, $$sanitizeUriProvider) {
 
         listeners.push(fn);
         $rootScope.$evalAsync(function() {
-          if (!listeners.$$inter && attrs.hasOwnProperty(key)) {
+          if (!listeners.$$inter && attrs.hasOwnProperty(key) && !isUndefined(attrs[key])) {
             // no one registered attribute interpolation function, so lets call it manually
             fn(attrs[key]);
           }

test/ng/compileSpec.js

@@ -2991,6 +2991,23 @@ describe('$compile', function() {
     );
 
 
+    it('should call observer only when the attribute value changes', function() {
+      module(function() {
+        directive('observingDirective', function() {
+          return {
+            restrict: 'E',
+            scope: { someAttr: '@' }
+          };
+        });
+      });
+      inject(function($rootScope, $compile) {
+        $compile('<observing-directive observer></observing-directive>')($rootScope);
+        $rootScope.$digest();
+        expect(observeSpy).not.toHaveBeenCalledWith(undefined);
+      });
+    });
+
+
     it('should delegate exceptions to $exceptionHandler', function() {
       observeSpy = jasmine.createSpy('$observe attr').andThrow('ERROR');