Use @> operator to speed up eq queries

Using @> would take advantage of GIN or GIST indexes to speed up eq query

```
dev=# explain select * from products where properties @> hstore('list_price', '3');
                                           QUERY PLAN
------------------------------------------------------------------------------------------------
 Bitmap Heap Scan on products  (cost=94.95..8976.68 rows=2841 width=83)
   Recheck Cond: (properties @> '"list_price"=>"3"'::hstore)
   ->  Bitmap Index Scan on index_products_on_properties  (cost=0.00..94.24 rows=2841 width=0)
         Index Cond: (properties @> '"list_price"=>"3"'::hstore)
(4 rows)

dev=# explain select * from products where properties -> 'list_price' = '3';
                            QUERY PLAN
------------------------------------------------------------------
 Seq Scan on products  (cost=0.00..83396.12 rows=14206 width=83)
   Filter: ((properties -> 'list_price'::text) = '3'::text)
(2 rows)
```