grunt.task.isTaskAlias not working

It's always returning `false`. So, I ran some tests:

``` javascript
grunt.registerTask("test", "", function(){});

grunt.registerTask("bleh", ["test"]);
grunt.registerTask("default", ["test"]);

console.log( grunt.task._tasks["bleh"].fn.alias ); // undefined
console.log( grunt.task._tasks["bleh"].info ); // "Alias for..."

console.log( grunt.task._tasks["default"].fn.alias ); // undefined
console.log( grunt.task._tasks["default"].info ); // "Alias for..."

console.log( this.task._tasks["default"].fn.alias ); // undefined
console.log( this.task._tasks["default"].info ); // "Alias for..."
```

Start by reproducing the behavior with the registerTask calls in the issue, then inspect grunt.task.isTaskAlias and the entries created in grunt.task._tasks. Compare an alias such as "bleh" or "default" with the ordinary "test" task. Done means alias tasks are identified consistently with the documented purpose of isTaskAlias.

Written by the indexing model from the issue text.