Angular update model from directive

HTML

<script src="http://code.angularjs.org/1.1.0/angular.min.js"></script>
<div ng-app="App">
    <div ng-controller="Ctrl" ng-init="init()">
        <form name="myForm">
            <input type="text" name="field1" ng-model="field1" http-prefix>
        </form>
    </div>   
</div>

JavaScript

var app = angular.module('App', []);

function Ctrl($scope) {
    $scope.init = function() {
        $scope.field1 = "dummy";
    }
}

app.directive('httpPrefix', function() {
    return {
        restrict: 'E',
        scope: {
            ngModel: '='
        },
        link: function(scope, element, attrs, controller) {
            element.bind('change', function() {
                scope.$apply(function() {
                    scope.ngModel = 'http://' + scope.ngModel;
                });
            });
        }
    };
});