Use print_r() to stringify cart session

Instead of json_encode(), because json_encode() takes types into
account, meaning numbers may end up being represented in the
encoded JSON as a string, e.g. "1" instead of 1, which can lead
to the md5() being different between requests.

This issue surfaces when creating a cart hash in a different
request to the request to pay for the order, because after adding
line items to the cart, the items quantity will be encoded with
json_encode() as an int, but when encoding them in a different
request, where they are coming from the database, they will be
encoded as a string.

includes/class-wc-checkout.php

@@ -184,7 +184,7 @@ public function create_order() {
 				'status'        => apply_filters( 'woocommerce_default_order_status', 'pending' ),
 				'customer_id'   => $this->customer_id,
 				'customer_note' => isset( $this->posted['order_comments'] ) ? $this->posted['order_comments'] : '',
-				'cart_hash'     => md5( json_encode( WC()->cart->get_cart_for_session() ) . WC()->cart->total ),
+				'cart_hash'     => md5( print_r( WC()->cart->get_cart_for_session(), true ) . WC()->cart->total ),
 				'created_via'   => 'checkout'
 			);