Add pre-orders support to gateway.

[peterwilsoncc]

The Pre-orders extension requires gateways to include support for the pre-orders feature in order for a gateway to be available for pay upon release orders using gateways that support tokenization.

As the dummy gateway doesn't include support for the feature, it's not possible to use it while testing tokenized pre-orders.

Adding it is a pretty simple fix by adding the feature within the WC_Gateway_Dummy constructor.