Encode WriteMultipleRegistersRequest using values length (#102)

Encode the "Byte Count" field of a WriteMultipleRegistersRequest using the length of the `values` bytes rather than multiplying `quantity` by 2.

This allows the request to be used with non-standard Modbus implementations e.g. those that use 32-bit registers instead of the standard 16-bit registers.

modbus/src/main/java/com/digitalpetri/modbus/pdu/WriteMultipleRegistersRequest.java

@@ -69,8 +69,7 @@ public static void encode(WriteMultipleRegistersRequest request, ByteBuffer buff
       buffer.putShort((short) request.address);
       buffer.putShort((short) request.quantity);
 
-      int byteCount = request.quantity * 2;
-      buffer.put((byte) byteCount);
+      buffer.put((byte) request.values.length);
       buffer.put(request.values);
     }