fix(ngClass): update if value set before first digest

Author: vojtajina

lib/directives/ng_class.dart

@@ -13,7 +13,7 @@ class NgClassAttrDirective {
         currentSet = current.split(' ');
       }
 
-      if (previous == null) {
+      if (previous == null || previous == current /* first digest */) {
         previousSet = [];
       } else {
         previousSet = previous.split(' ');

test/directives/ng_class_spec.dart

@@ -7,9 +7,10 @@ main() {
     var compile, element, rootScope;
 
     beforeEach(inject((Scope scope, Compiler compiler, Injector injector) {
+      rootScope = scope;
+
       compile = (html, [applyFn]) {
         element = $(html);
-        rootScope = scope;
         compiler(element)(injector, element);
         scope.$apply(applyFn);
       };
@@ -75,5 +76,12 @@ main() {
       expect(element).toHaveClass('third');
       expect(element).toHaveClass('original');
     });
+
+
+    it('should update value that was set before compilation', () {
+      rootScope['active'] = 'something';
+      compile('<div ng-class="active"></div>');
+      expect(element).toHaveClass('something');
+    });
   });
 }