fix (and test) for encoding submit_sm_resp messages with no message_id

src/main/java/com/cloudhopper/smpp/pdu/BaseSmResp.java

@@ -63,7 +63,14 @@ public int calculateByteSizeOfBody() {
 
     @Override
     public void writeBody(ChannelBuffer buffer) throws UnrecoverablePduException, RecoverablePduException {
-        ChannelBufferUtil.writeNullTerminatedString(buffer, this.messageId);
+        // when this PDU was parsed, it's possible it was missing the messageId instead
+        // of having a NULL messageId. If that's the case, the commandLength will be just
+        // enough for the headers (and theoretically any optional TLVs). Don't try to
+        // write the NULL byte for that case.
+        // See special note in 4.4.2 of SMPP 3.4 spec
+        if (!((buffer.writableBytes() == 0) && (this.messageId == null))) {
+            ChannelBufferUtil.writeNullTerminatedString(buffer, this.messageId);
+        }
     }
 
     @Override

src/test/java/com/cloudhopper/smpp/transcoder/PduEncoderTest.java

@@ -82,14 +82,25 @@ public void encodeSubmitSmResp() throws Exception {
     }
 
     @Test
-    public void encodeSubmitSmRespWithNoMessageId() throws Exception {
+    public void encodeSubmitSmRespWithNullMessageId() throws Exception {
         SubmitSmResp pdu0 = new SubmitSmResp();
         pdu0.setSequenceNumber(171192033);
 
         ChannelBuffer buffer = transcoder.encode(pdu0);
         Assert.assertArrayEquals(HexUtil.toByteArray("0000001180000004000000000a342ee100"), BufferHelper.createByteArray(buffer));
     }
 
+    @Test
+    public void encodeSubmitSmRespWithOmittedMesageId() throws Exception {
+        SubmitSmResp pdu0 = new SubmitSmResp();
+        pdu0.setSequenceNumber(171192033);
+        pdu0.setCommandStatus(0x30);
+        pdu0.setCommandLength(16);
+
+        ChannelBuffer buffer = transcoder.encode(pdu0);
+        Assert.assertArrayEquals(HexUtil.toByteArray("0000001080000004000000300a342ee1"), BufferHelper.createByteArray(buffer));
+    }
+
     @Test
     public void encodeDeliverSmResp() throws Exception {
         DeliverSmResp pdu0 = new DeliverSmResp();