Enhancement: Properly handle discount_code so its automatically added to $data sent to Zapier

[toodlepip]

When data is constructed in class-pmpro-zapier.php from the MemberOrder object, the discount code is not properly added to the $data array that is then sent to Zapier.

When the order is loaded it's handled in the code below for all of the different Zapier options available:

static function pmpro_added_order( $order ) {
		// bail if setting is not checked
		$options = PMPro_Zapier::get_options();
		if ( empty( $options['pmpro_added_order'] ) ) {
			return;
		}

		// Get the saved order.
		$order = new MemberOrder( $order->id );

But this code loads in the DiscountCode object into $order, which is then set to NULL when passed through to Zapier.

The additional of this line will retrieve the discount code used and replace the discount_code object:

$order->discount_code = $order->getDiscountCode()->code;

This will then be available from Zapier to assign to the target application.