Only rewind the current container in SdBusMessage.get_contents

Otherwise properties break because the actual property set message
consists of tuple of interface and property name and variant of
value. The libsystemd will place the cursor to only read the
value from message but completely rewinding message will point
again to the tuple.

src/sdbus/sd_bus_internals_message.c

@@ -955,7 +955,7 @@ static PyObject* iter_tuple_or_single(_Parse_state* parser) {
 }
 
 static PyObject* SdBusMessage_get_contents2(SdBusMessageObject* self, PyObject* Py_UNUSED(args)) {
-        const char* message_signature = sd_bus_message_get_signature(self->message_ref, 1);
+        const char* message_signature = sd_bus_message_get_signature(self->message_ref, 0);
 
         if (message_signature == NULL) {
                 PyErr_SetString(PyExc_TypeError, "Failed to get message signature.");
@@ -966,7 +966,7 @@ static PyObject* SdBusMessage_get_contents2(SdBusMessageObject* self, PyObject*
                 Py_RETURN_NONE;
         }
 
-        CALL_SD_BUS_AND_CHECK(sd_bus_message_rewind(self->message_ref, 1));
+        CALL_SD_BUS_AND_CHECK(sd_bus_message_rewind(self->message_ref, 0));
         _Parse_state read_parser = {
             .message = self->message_ref,
             .container_char_ptr = message_signature,

test/test_read_write_dbus_types.py

@@ -435,7 +435,10 @@ def test_reading_multiple_times(self) -> None:
         message.seal()
 
         for _ in range(5):
-            message.get_contents()
+            self.assertEqual(
+                message.get_contents(),
+                "test",
+            )
 
 
 if __name__ == "__main__":