Add a core_test for this functionality.

Author: theengineear

plotly/tests/test_core/test_utils/__init__.py

@@ -0,0 +1,3 @@
+def setup_package():
+    import warnings
+    warnings.filterwarnings('ignore')

plotly/tests/test_core/test_utils/test_utils.py

@@ -0,0 +1,13 @@
+import json
+from unittest import TestCase
+
+from plotly.utils import PlotlyJSONEncoder
+
+
+class TestJSONEncoder(TestCase):
+
+    def test_nan_to_null(self):
+        array = [1, float('NaN'), float('Inf'), float('-Inf'), 'platypus']
+        result = json.dumps(array, cls=PlotlyJSONEncoder)
+        expected_result = '[1, null, null, null, "platypus"]'
+        self.assertEqual(result, expected_result)