Default :state to n/a for NetworkMerchants

NetworkMerchants requires a `:state` to be passed otherwise it will error with ` The state field is required`.

This defaults the `:state` to `n/a` when not provided. Looks like we have the same issue with other gateways: https://github.com/Shopify/active_merchant/blob/a6cfae5e77ed249de84dbe46a4a91e3a0903a11d/lib/active_merchant/billing/gateways/secure_net.rb#L153

### Review

@fw42 @jduff 

/cc @mackiec @louiskearns