[FIX]: copied data should be const

Context:

In order to store data in `UserDefault`, I use this:

```cpp
data.copy(reinterpret_cast<const unsigned char *>(this), sizeof(User));
```

Since `this` is a const pointer, I either have to use `const_cast` with the current API, which is bad, or change it, so I decided to change it.

Note, this only makes the API more strict within the method, so of course it is compatible with non const values.

Thanks,