[BUG] Modified Symbol Instance reverts to base symbol on save and reopen

[matijaabicic]

1. Create a symbol (say, rectangle and some text)
2. Add instance of a symbol to an artboard
3. Modify instance (change text)
4. Save file
5. Close Lunacy
6. Open the file
   – instance of the symbol lost the changes and has reverted to original symbol text