Sum two values of a single cell, group by dates, and return 0 for dates where there are no results

gistfile1.sql

SELECT year, month, sum(total) as total, sum(deliverCount) as delivered, sum(rejectCount) as rejected
FROM (
  SELECT extract(year from created) AS year, extract(month from created) AS month, count(*) as total,
    sum(CASE WHEN action = 'deliver' then 1 else 0 end) deliverCount,
    sum(CASE WHEN action = 'discard' then 1 else 0 end) rejectCount
  FROM transfer
  WHERE subscriber_id = 1
  AND created BETWEEN '2015-01-01 0:0' AND '2016-01-01 0:0'
  GROUP BY extract(year from created), extract(month from created)

  UNION
    SELECT extract(year FROM generate_series('2015-01-01 0:0'::TIMESTAMP, '2015-12-31 0:0'::TIMESTAMP, INTERVAL '1 month')) AS year,
       extract(month FROM generate_series('2015-01-01 0:0'::TIMESTAMP, '2015-12-31 0:0'::TIMESTAMP, INTERVAL '1 month')) AS month,
       0, 0, 0
) AS t
GROUP BY year, month
ORDER BY year, month;