Postgres Reverse change missing comma

[tprebs]

I noticed as part of using the new ent versioned migrations that all of the down scripts with column modifications were not adding commas in correctly

Up generates correctly e.g.

ALTER TABLE "subscriptions" ALTER COLUMN "created_at" SET DEFAULT CURRENT_TIMESTAMP, ALTER COLUMN "updated_at" SET DEFAULT CURRENT_TIMESTAMP;

Down has missing commas between each column change

ALTER TABLE "subscriptions" ALTER COLUMN "created_at" DROP DEFAULT ALTER COLUMN "updated_at" DROP DEFAULT;

[a8m]

Closed with #668

Thanks for the contribution 🙏