xdr: introduce argument-less constructor for XdrOpaque

Motivation:
To use XdrOpaque as reply from an RPC call we need to create an instance
of XdrOpaque without arguments:

XdrAble arg = ...;
XdrOpaque reply = new XdrOpaque();
clnt.call(0, arg, reply);
  

Modification:
introduce argument-less constructor for XdrOpaque

Result:
XdrOpaque can be used as reply of rpc request.

Acked-by: Paul Millar
Target: master

oncrpc4j-core/src/main/java/org/dcache/oncrpc4j/xdr/XdrOpaque.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2009 - 2018 Deutsches Elektronen-Synchroton,
+ * Copyright (c) 2009 - 2019 Deutsches Elektronen-Synchroton,
  * Member of the Helmholtz Association, (DESY), HAMBURG, GERMANY
  *
  * This library is free software; you can redistribute it and/or modify
@@ -33,6 +33,9 @@ public class XdrOpaque implements XdrAble {
 
     private byte[] _opaque;
 
+    public XdrOpaque() {
+    }
+
     public XdrOpaque(byte[] opaque) {
         _opaque = opaque;
     }

oncrpc4j-core/src/test/java/org/dcache/oncrpc4j/xdr/XdrOpaqueTest.java

@@ -100,6 +100,24 @@ public void testDecodeWellKnown() throws IOException {
         }
     }
 
+    @Test
+    public void testDecodeWellKnownStandardConstructor() throws IOException {
+
+        byte[] data = new byte[]{
+            0x0, 0x0, 0x0, 0x08, // size
+            0x0C, 0x0A, 0x0F, 0x0E, 0x0B, 0x0A, 0x0B, 0x0E // data
+        };
+
+        try ( Xdr xdr = new Xdr(data)) {
+            xdr.beginDecoding();
+
+            XdrOpaque opaque = new XdrOpaque();
+            opaque.xdrDecode(xdr);
+            // the data part must match (e.g without size)
+            assertArrayEquals(Arrays.copyOfRange(data, 4, 12), opaque.getOpaque());
+        }
+    }
+
     @Test
     public void testEncodeWellKnown() throws IOException {