[FIX] allow postgres_set variables to be reused.
[FIX] allow postgres_set variables to be reused.

This allow using the same code snippet to be included in multiple `server` directives.

Fixes FRiCKLE/ngx_postgres#17

lightcn/ngx_postgres,lightcn/ngx_postgres,nagyistoce/odoo-ngx_postgres,nagyistoce/odoo-ngx_postgres