[BUGFIX beta] Correct type of variable initialization

`allowedKeys` should be a string value because it will be passed from template.

Such as:
``` handlebars
<a href="#" {{action "edit" allowedKeys="alt"}}>click me</a>
```

If it is treated as an array, [`allowedKeys.indexOf`][] doesn't work on old IE.
Because `Array#indexOf` is not defined in old IE.

[`allowedKeys.indexOf`]: https://github.com/emberjs/ember.js/blob/a51806a2cdc2fc93f8ed3fe6d0d53ab229c455d3/packages/ember-routing/lib/helpers/action.js#L48
Merge pull request #3730 from tricknotes/fix-for-ie8

[BUGFIX] Correct type of variable initialization