Tests `self.style_function` and `self.highlight_function` to ensure
        they are functions returning dictionaries.

def _validate_function(self, func, name):
        """
        Tests `self.style_function` and `self.highlight_function` to ensure
        they are functions returning dictionaries.
        """
        test_feature = self.data['features'][0]
        if not callable(func) or not isinstance(func(test_feature), dict):
            raise ValueError('{} should be a function that accepts items from '
                             'data[\'features\'] and returns a dictionary.'
                             .format(name))