return symbolized keys when using results_as_hash = true

Currently returning thousands of records with string instances as keys. Seems wasteful.

``` ruby
[12] pry(main)> DB.execute("select * from politicians")[0].keys.first.object_id # is String: "name"
=> 70139106472200
[13] pry(main)> DB.execute("select * from politicians")[1].keys.first.object_id # is String: "name"
=> 70139116927900
```
