Tag searches on composite indexes is not working properly

```
I have composite indexer like
composite_index = CompositeIndexer(SomeModel.indexer, AnotherModel.indexer)

and I am trying to get result in the following way.

composite_index.search("name:abx"). 

I am having 'name' tag into one of the indexer. But its returning a resultset 
with 0 length.
```

Original issue reported on code.google.com by `nikhil.n...@gmail.com` on 12 Sep 2013 at 10:43
