Allow MapObject to copy field types that are mapped inside the ObjectMapping

[AeonLucid]

For my Il2CppInspector loader I had to add a mapping for Il2CppAssemblyDefinition and Il2CppAssemblyNameDefinition.

public class Il2CppAssemblyDefinition
{
    // ..

    public Il2CppAssemblyNameDefinition aname_post16;
}

public class Il2CppAssemblyNameDefinition
{
    // ..
}

stream.AddObjectMapping(typeof(Il2CppAssemblyDefinition), typeof(Types.Il2CppAssemblyDefinition));
stream.AddObjectMapping(typeof(Il2CppAssemblyNameDefinition), typeof(Types.Il2CppAssemblyNameDefinition));

Without this pull request, this would fail with the exception Object must implement IConvertible because it can not cast between two different types on Convert.ChangeType.