Network Merchants - Added remote tests and cleaned up code

- Added remote tests for track data / fixed one
- Refactored some code network_merchants so useless data is not sent
- All remote tests pass

@j-mutter @jnormore @louiskearns 
@j-mutter likes to tell me that [`creditcard_or_vault_id.track_data.present?`](https://github.com/Shopify/active_merchant/blob/master/lib/active_merchant/billing/gateways/usa_epay_transaction.rb#L149) should suffice here.