Test that extra_data can be encoded and decoded (#27)

* Test that extra_data can be encoded and decoded

* I had one line and I messed it up

vcap/vcap/testing/input_output_validation.py

@@ -9,6 +9,7 @@
 import cv2
 import mock
 import numpy as np
+import json
 
 from vcap import (
     NodeDescription,
@@ -156,6 +157,12 @@ def _run_inference_on_images(images: List[np.ndarray], capsule: BaseCapsule):
                     ("Capsule failed to output a prediction that matches "
                      "the NodeDescription it had for it's output type. "
                      f"Prediction: {prediction}")
+
+                # Assert the nodes "extra_data" attribute can be JSON encoded
+                # without errors. Typically this can happen if there's a numpy
+                # array carelessly left in the extra_data
+                json.loads(json.dumps(output_node.extra_data))
+
                 # If this capsule can encode things, verify that the backend
                 # correctly implemented the "distance" function
                 if (capsule.capability.encoded