csharp: Provisionally fix conversion of eina.Value

csharp: Provisionally fix conversion of eina.Value

When we have an eina.Value_Native (representing an Eina_Value passed by
value) and assign it to an eina.Value (a class with an IntPtr to an
underlying Eina_Value) we copy it so the eina.Value can take ownership
and free the data normally.

A possibly better alternative would be adding an extra flag to
eina.Value (something like OwnsPointer) to check whether we should free
the struct we point to or not.