Fix workflow preview under Python 3

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

mayan/apps/document_states/tests/test_models.py

@@ -0,0 +1,11 @@
+from __future__ import unicode_literals
+
+from mayan.apps.common.tests import BaseTestCase
+
+from .mixins import WorkflowTestMixin
+
+
+class WorkflowModelTestCase(WorkflowTestMixin, BaseTestCase):
+    def test_workflow_template_preview(self):
+        self._create_test_workflow()
+        self.assertTrue(self.test_workflow.get_api_image_url())