Add support for JSONB.

[rafaelveloso]

pl/v8 can't seem to properly handle JSONB.

Test case

Having the following function:

CREATE OR REPLACE FUNCTION jsonb_test(data anyelement)
  RETURNS jsonb AS
    $BODY$
       return JSON.stringify(data);
    $BODY$
LANGUAGE plv8 STABLE STRICT;

When running:

SELECT jsonb_test('{"a":1, "b":{"c": 2}}'::json);
result: {"a": 1, "b": {"c": 2}}

will result in {"a": 1, "b": {"c": 2}} which is the expected result.

When running

SELECT jsonb_test('{"a":1, "b":{"c": 2}}'::jsonb);

we get "{\"a\": 1, \"b\": {\"c\": 2}}", which is not ideal.

[trailsandtribulations]

some notes:

1. SELECT jsonb_test('{"a":1, "b":{"c": 2}}'::json) works fine
2. the type jsonb seems to be internal to the pg database, that is, not stored as a string but parsed, so don't see quite how {}::jsonb as an argument makes sense
3. that said, there appears to be a bug, nonetheless, plv8 claims to support json, not jsonb.
4. I don't see the pg engine supporting json in any fashion except for text, so it seems to me nothing gained anywhere by typing something to jsonb within a javascript function.

[rafaelveloso]

There is a recently merged PR regarding JSONB support for plv8
plv8/plv8#116