chore(rating): remove redundant validation and add unit tests

Closes angular-ui#3464

Author: imagentleman

src/rating/rating.js

@@ -81,10 +81,7 @@ angular.module('ui.bootstrap.rating', [])
     replace: true,
     link: function(scope, element, attrs, ctrls) {
       var ratingCtrl = ctrls[0], ngModelCtrl = ctrls[1];
-
-      if ( ngModelCtrl ) {
-        ratingCtrl.init( ngModelCtrl );
-      }
+      ratingCtrl.init( ngModelCtrl );
     }
   };
 });

src/rating/test/rating.spec.js

@@ -165,6 +165,17 @@ describe('rating directive', function () {
       expect($rootScope.rate).toBe(3);
     });
 
+    it('supports only arrow keys', function() {
+      $rootScope.rate = undefined;
+      $rootScope.$digest();
+
+      triggerKeyDown(36);
+      expect($rootScope.rate).toBe(undefined);
+
+      triggerKeyDown(41);
+      expect($rootScope.rate).toBe(undefined);
+    });
+
     it('can get zero value but not negative', function() {
       $rootScope.rate = 1;
       $rootScope.$digest();