Added a test for colorscales specified as a string

Added a test checking that a colorscale specified as a string is returned as a string. Previously it had been returned as a tuple of 1-tuples. e.g. "Viridis" -> (('V',), ('i',), ('r',), ('i',), ('d',), ('i',), ('s',)). Catches plotly#1087.

plotly/tests/test_core/test_graph_objs/test_properties_validated.py

@@ -102,6 +102,16 @@ def test_present_colorscale(self):
         # Presented as tuple of tuples
         self.assertEqual(self.scatter.marker.colorscale,
                          ((0, 'red'), (1, 'green')))
+
+    def test_present_colorscale_str(self):
+        self.assertIsNone(self.scatter.marker.colorscale)
+
+        # Assign string
+        self.scatter.marker.colorscale = "Viridis"
+
+        # Presented as a string
+        self.assertEqual(self.scatter.marker.colorscale,
+                         "Viridis")
 
 
 class TestPropertyIterContains(TestCase):