Supports the case if the copy source does not start with a leading pa…

…th delimiter 🚑

Unlike the Java library, the `aws` CLI does not send a leading path delimiter, leading to an error further down the line.

This was reported by @gerhardj-b.

#230

src/main/java/ninja/S3Dispatcher.java

@@ -765,7 +765,8 @@ private void copyObject(WebContext webContext, Bucket bucket, String id, String
                               String.format("Source '%s' must contain '/'", copy));
             return;
         }
-        String srcBucketName = copy.substring(1, copy.indexOf(PATH_DELIMITER, 1));
+        int cutFrom = copy.startsWith(PATH_DELIMITER) ? PATH_DELIMITER.length() : 0;
+        String srcBucketName = copy.substring(cutFrom, copy.indexOf(PATH_DELIMITER, cutFrom));
         String srcId = copy.substring(copy.indexOf(PATH_DELIMITER, 1) + 1);
         Bucket srcBucket = storage.getBucket(srcBucketName);
         if (!srcBucket.exists()) {