fix(date): Prevent jquery datepicker formatDate exception

fixes #48

.travis.yml

@@ -11,3 +11,7 @@ before_install:
 
 #script: "grunt"
 sudo: false
+
+cache:
+  directories:
+    - node_modules

src/date.js

@@ -27,7 +27,11 @@ if (typeof module !== 'undefined' && typeof exports !== 'undefined' && module.ex
         dateFormat = dateFormat || uiDateFormatConfig;
         if (value) {
           if (dateFormat) {
-            return jQuery.datepicker.formatDate(dateFormat, value);
+            try {
+              return jQuery.datepicker.formatDate(dateFormat, value);
+            } catch (formatException) {
+              return undefined;
+            }
           }
 
           if (value.toISOString) {

test/date.spec.js

@@ -411,6 +411,17 @@ describe('uiDateFormat', function() {
       });
     });
 
+    it('should not throw when a user types in an incomplete value', function() {
+      inject(function($compile, $rootScope) {
+        var element = $compile('<input ui-date-format="yy-mm-dd" ng-model="x"/>')($rootScope);
+        var ngModel = element.controller('ngModel');
+        expect(function incompleteValue() {
+          ngModel.$setViewValue('2015-');
+        }).not.toThrow();
+        expect(ngModel.$modelValue).toBeUndefined();
+      });
+    });
+
     it('should convert empty strings to null', inject(function($compile, $rootScope) {
       var element = $compile('<input ui-date-format ng-model="x">')($rootScope);
       element.controller('ngModel').$setViewValue('');