Allow for cloning the cart object

This handles creating new instances of the `$session` and `$fees_api` properties.

includes/class-wc-cart.php

@@ -122,6 +122,15 @@ class WC_Cart extends WC_Legacy_Cart {
 		add_action( 'woocommerce_after_checkout_validation', array( $this, 'check_customer_coupons' ), 1 );
 	}
 
+	/**
+	 * When cloning, ensure object properties are handled.
+	 */
+	public function __clone() {
+		// These properties store a reference to the cart, so just use new instead of clone.
+		$this->session = new WC_Cart_Session( $this );
+		$this->fees_api = new WC_Cart_Fees( $this );
+	}
+
 	/*
 	|--------------------------------------------------------------------------
 	| Getters.