Casting hstore to json

Hi.

I'm trying to cast hstore to json, but I don't seem to be getting a json
object from hstore key/value pairs.

9.3 documentation says:
*Note:* The hstore
<http://www.postgresql.org/docs/9.3/static/hstore.html> extension
has a cast from hstore to json, so that converted hstore values are
represented as JSON objects, not as string values.
to_json(anyelement) - If the data type is not built in, and there is a cast
from the type tojson, the cast function will be used to perform the
conversion.

On 9.3.5 server:
db=> SELECT '"a"=>1, "b"=>2'::hstore;
hstore
--------------------
"a"=>"1", "b"=>"2"
(1 row)

db=> SELECT to_json('"a"=>1, "b"=>2'::hstore);
to_json
------------------------------
"\"a\"=>\"1\", \"b\"=>\"2\""
(1 row)
db=> SELECT cast(('"a"=>1, "b"=>2'::hstore) as json);
ERROR: cannot cast type hstore to json
LINE 1: SELECT cast(('"a"=>1, "b"=>2'::hstore) as json);

Thanks!
Pawel.