fix(datepicker): default to current date

- When model is empty, default to current date

Closes angular-ui#5395
Fixes angular-ui#5190

src/datepicker/datepicker.js

@@ -842,7 +842,7 @@ function(scope, element, attrs, $compile, $parse, $document, $rootScope, $positi
       ngModel.$parsers.unshift(parseDate);
       ngModel.$formatters.push(function(value) {
         if (ngModel.$isEmpty(value)) {
-          scope.date = value;
+          scope.date = new Date();
           return value;
         }