allow to use several values in a nginx directive

For instance if I need to use the proxy_set_header multiple times to get a nginx config file like this:

location / {
  proxy_pass          http://segundo_precio_app;
  proxy_read_timeout  90;
  proxy_set_header    Host $host
  proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;
}

This patch allows an array so now I can puppet definition as:

location_cfg_append => {
  proxy_set_header => [
   'Host $host',
   'X-Forwarded-For $proxy_add_x_forwarded_for'
  ],
}