webdav: fix range header formatting in relay request

An open-ended range like "bytes=0-" produced an invalid header
like "bytes=0-null", resulting in no data being transferred. This
patch implements support for open-ended ranges (bytes=0-) and
suffix ranges (bytes=-500).

References #7971

Author: mzur

modules/dcache-webdav/src/main/java/org/dcache/webdav/DcacheResourceFactory.java

@@ -1799,8 +1799,19 @@ public void relayData(OutputStream outputStream, io.milton.http.Range range)
                 try {
                     connection.setRequestProperty("Connection", "Close");
                     if (range != null) {
-                        connection.addRequestProperty("Range",
-                              String.format("bytes=%d-%d", range.getStart(), range.getFinish()));
+                        String rangeHeader;
+                        Long start = range.getStart();
+                        Long finish = range.getFinish();
+
+                        if (start == null && finish != null) {
+                            rangeHeader = String.format("bytes=-%d", finish);
+                        } else if (start != null && finish == null) {
+                            rangeHeader = String.format("bytes=%d-", start);
+                        } else {
+                            rangeHeader = String.format("bytes=%d-%d", start, finish);
+                        }
+
+                        connection.addRequestProperty("Range", rangeHeader);
                     }
 
                     connection.connect();