JSON array inputs may need explicit cast/stringify

[aleclarson]

If you're calling a Postgres function that accepts t.json or t.jsonb and you pass a JS array, it will be serialized as a Postgres array (not a JSON array). This is a limitation of node-postgres. It also affects INSERT queries, but the runtime types I'm adding for #14 will fix that in the next alpha version.

Related to brianc/node-postgres#2012