Serialization/deserialization doesn't work

[paulkaplan]

To reproduce.

1. Load https://llk.github.io/scratch-gui/
2. Add the "brick wall" backdrop (or add any sprite / costume)
3. Click "Save"
4. Reload https://llk.github.io/scratch-gui/
5. Click "Load" and choose the file you just downloaded
6. Doesn't load, there is an error in the console

gui.js:146209 GET https://cdn.assets.scratch.mit.edu/internalapi/asset/4f234884958b11023d44abc010ad50ec.undefined/get/

That .undefined certainly looks suspicious. Probably has something to do with the storage change?

[DanFu09]

This happens because the dataFormat field introduced in scratchfoundation/scratch-storage#12 by @cwillisf doesn't get saved to the JSON file. I'm not sure where that happens, but adding that field in should fix it. Relatedly, it seems the loading code doesn't correctly handle the case where the dataformat isn't known (the "default" behavior).