`insert_or_assign` fails for rvalues

```c++
dynamic_string key("key");

dynamic_string val1("val1");
dynamic_string val2("val2");
// ...
map.insert(key, val1); // inserts ("key", "val1")
map.insert_or_assign(key, move(val2)); // inserts ("key", "")
```

the value is first moved into a tuple, which is destroyed if the insertion fails,
so then the follow-up assignment assigns an empty string