Support writing into native Postgres array columns

[motiz88]

Related to PostgREST/postgrest#501. To match PostgREST's behavior, jsgrest sends array values to the server as JSON ([1,2,3]), never as Postgres array literals ({1,2,3}). This only works when writing to json/jsonb columns and fails when writing to int[] etc.

I hope this can be solved without foreknowledge of the schema but am not sure. Note to self: look into libpqtypes.