nn.loadData() appears to be broken

[gohai]

When loading a JSON file produced by nn.saveData(), the following error message is returned (with the latest 1.x version of the library): data must be a json object containing an array called "data"

To reproduce: try pressing l in this sketch. The data3.json file has been produced using the same code (same ml5 version).

[ziyuan-linn]

@gohai Thank you for submitting this issue! Looks like there was an incorrect call to JSON.stringify on an already stringified JSON object. I opened a PR to fix this.