xdr: do not flip byte buffer in Xdr#xdrEncodeByteBuffer

Motivation:
The most of java API that ByteBuffer consumers use it
without flipping. It make sense to make Xdr#xdrEncodeByteBuffer to
follow that behavior to offer familiar behavior to application
developers.

Modification:
do not flip byte buffer in Xdr#xdrEncodeByteBuffer

Result:
More standard behavior.

Fixes: #62
Acked-by: Paul Millar
Target: master, 3.0

README.md

@@ -146,6 +146,20 @@ org.dcache.xdr.GrizzlyXdrTransport => into org.dcache.oncrpc4j.grizzly.GrizzlyRp
 
 org.dcache.xdr.XdrBuffer is removed. Use org.dcache.oncrpc4j.xdr.Xdr.
 
+### Behavoir change ###
+
+The Xdr#xdrEncodeByteBuffer changed to not flip provided byte buffer. As a result, the Xdr#xdrEncodeByteBuffer
+will encode data in the buffer from buffers current position up to the limit:
+
+```
+ButeByffer buffer = ...;
+Xdr xdr = ...;
+
+buffer.put(...);
+buffer.flip();
+xdr.xdrEncodeByteBuffer(buffer);
+```
+
 
 Using RPCGEN to generate client and server stubs
 ------------------------------------------------

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

@@ -656,9 +656,14 @@ public void xdrEncodeLong(long value) {
        _buffer.putLong(value);
     }
 
+    /**
+     * Encodes (aka "serializes") a sequence of bytes from the given buffer
+     * to this Xdr stream.
+     *
+     * @param buf The buffer from which bytes are to be retrieved.
+     */
     @Override
     public void xdrEncodeByteBuffer(ByteBuffer buf) {
-        buf.flip();
         int len = buf.remaining();
         int padding = (4 - (len & 3)) & 3;
         xdrEncodeInt(len);