Hotfix: Batch label creation merges wrong shipping labels

Within a bulk action each labels will be downloaded
and merged into one big PDF.
Some orders already have labels created manually (via edit screen),
so unfortunately a merge contains such "old" labels too.
Therefor the caching of already created labels needs a proper hash
to separate new and old ones even across API versions.

- Hash of URL covers more scenarios than the described one.
- Pay attention that this can lead to much obsolete files over time.
- see github #125 for more

components/woo/order-bulk.php

@@ -165,7 +165,7 @@ protected function create_label( $request ) {
 	 */
 	protected function save_label( $order_id, $url ) {
 		$path = $this->get_storage_path( 'order' . DIRECTORY_SEPARATOR . $order_id )
-		        . DIRECTORY_SEPARATOR . 'label.pdf';
+		        . DIRECTORY_SEPARATOR . md5( $url ) . '.pdf';
 
 		if ( file_exists( $path ) ) {
 			// Might be already downloaded, so we won't overwrite it.