Read a body from message_payload and accept data_sm (#84)

* Regression tests for a body in message_payload and for data_sm

* Read a body from message_payload and accept data_sm

* Assert the fixed message_payload and data_sm behaviour against Jasmin

* Record the message_payload and data_sm fixes in the Jasmin findings

* Read an inbound data_sm as the direction it travelled, and export messageOctets

* Refuse a segment with the code its stand-in command defines

* Name the stand-in the refusal status is read from

src/message-body.ts

@@ -0,0 +1,17 @@
+import type { PduObject } from './pdu.ts';
+
+/**
+ * The user data, wherever the peer put it. SMPP 3.4 5.3.2.32 carries up to 64 KB in
+ * `message_payload` with `sm_length` 0, and a `data_sm` has nowhere else to put a body at all.
+ * A filled `short_message` is the peer using the mandatory field, so it wins over a TLV the spec
+ * only allows in its place.
+ */
+export function messageOctets(pduObj: PduObject): Buffer | undefined {
+	const shortMessage = pduObj.shortMessageOctets;
+
+	if (shortMessage !== undefined && shortMessage.length > 0) return shortMessage;
+
+	const payload = pduObj.tlvs.message_payload?.tagValue;
+
+	return Buffer.isBuffer(payload) ? payload : shortMessage;
+}