`index_exists?` with `:name` checks specified columns.

[Yves Senn & Matthew Draper]

The column check was embodied in the defaul index name.
If the :name option was used, the specified columns were not verified at all.

Given:

```
assert connection.index_exists?(table_name, :foo_id, :name => :index_testings_on_yo_momma)
```

That index could have been defined on any field, not necessarily on `:foo_id`.