Add legit test for pdu encoding

pdu/submit.go

@@ -147,3 +147,7 @@ func (p *SUBMIT_SM) Size() int {
 	}
 	return size
 }
+func (p *SUBMIT_SM) UpdateSize() {
+	p.header.command_length = uint32(p.Size())
+	p.sm_length = byte(len(p.short_message))
+}