`forward_as_tuple` not working as expected

The current code for map insert is
```c++
template<typename K, typename V>
pair<iterator, bool> insert(K &&key, V &&val) 
{ return m_table.insert_unique(make_tuple(forward<K>(key), forward<V>(val))); }
```

but if you replace it with
```
return m_table.insert_unique(forward_as_tuple(key, val));
```

insertion will create duplicates for dynamic types like `dynamic_string`