Issues when passing multi-output model as argument

Hi there,
I got this error whenever I pass my model (converted in a foolbox model) as argument in accuracy() or attack().

```
/usr/local/lib/python3.6/dist-packages/foolbox/utils.py in accuracy(***failed resolving arguments***)
13 del inputs, labels
14
---> 15 predictions = fmodel(inputs_).argmax(axis=-1)
16 accuracy = (predictions == labels_).float32().mean()
17 return accuracy.item()

/usr/local/lib/python3.6/dist-packages/foolbox/models/base.py in __call__(self, inputs)
100 x, restore_type = ep.astensor_(inputs)
101 y = self._preprocess(x)
--> 102 z = ep.astensor(self._model(y.raw))
103 return restore_type(z)
104

/usr/local/lib/python3.6/dist-packages/eagerpy/astensor.py in astensor(x)
52 if name == "numpy" and isinstance(x, m[name].ndarray): # type: ignore
53 return NumPyTensor(x)
---> 54 raise ValueError(f"Unknown type: {type(x)}")
55
56

ValueError: Unknown type:
```

It's worth mentioning that my Keras model is not Sequential and has multiple outputs. When reducing the model output to one it goes without issues. Is that multi-output models must be set up differently or they are just not supported?