fix(alert): correct binding of alert type class

Regression from #1745 as there's a double interpolation expected but
ngClass doesn't support that.

The attribute value of ngClass is only interpolated once and the
interpolated expression is watched.

Fixes #2145
Closes #2146

src/alert/test/alert.spec.js

@@ -50,6 +50,16 @@ describe('alert', function () {
     expect(alerts.eq(2)).toHaveClass('alert-warning');
   });
 
+  it('should respect alert type binding', function () {
+    var alerts = createAlerts();
+    expect(alerts.eq(0)).toHaveClass('alert-success');
+
+    scope.alerts[0].type = 'error';
+    scope.$digest();
+
+    expect(alerts.eq(0)).toHaveClass('alert-error');
+  });
+
   it('should show the alert content', function() {
     var alerts = createAlerts();
 

template/alert/alert.html

@@ -1,4 +1,4 @@
-<div class="alert" ng-class="{'alert-{{type || 'warning'}}': true, 'alert-dismissable': closeable}" role="alert">
+<div class="alert" ng-class="['alert-' + (type || 'warning'), closeable ? 'alert-dismissable' : null]" role="alert">
     <button ng-show="closeable" type="button" class="close" ng-click="close()">
         <span aria-hidden="true">&times;</span>
         <span class="sr-only">Close</span>