[BUGFIX beta] uniq reduceComputed dependent keys.

If property brace expansion is enabled, there is never a need to have
duplicate dependent keys.  However, with proeprty brace expansion
disabled, it is the only way to specify multiple item property keys, as in
the following:

```js

var obj = Em.Object.createWithMixins({
 items: Em.A([{ foo: true }, { bar: false }, { bar: true }]),
 countFooOrBar: Em.reduceComputed({
   initialValue: 0,
   addedItem: function (acc, item) {
     if ( get(item, 'foo') || get(item, 'bar')) {
       ++acc;
     }
     return acc;
   },

    removedItem: function (acc, item) {
     if ( get(item, 'foo') || get(item, 'bar')) {
       --acc;
     }
     return acc;
   }
 }).property('items.@each.foo', 'items.@each.bar')
});
```

Of course, with property brace expansion, it would be better to write the
dependency as `property('items.@each.{foo,bar}')`.

Fix #3554.