BOM-618 Decode test client content.

The django test client returns bytes, and many of our tests start using
it like a string. This was fine in python 2 but not in python3.

lms/djangoapps/discussion/django_comment_client/tests/test_middleware.py

@@ -30,7 +30,7 @@ class AjaxExceptionTestCase(TestCase):
         self.assertEqual(self.exception1.status_code, response1.status_code)
         self.assertEqual(
             {"errors": json.loads(text_type(self.exception1))},
-            json.loads(response1.content)
+            json.loads(response1.content.decode('utf-8'))
         )
 
         response2 = self.a.process_exception(self.request1, self.exception2)
@@ -38,7 +38,7 @@ class AjaxExceptionTestCase(TestCase):
         self.assertEqual(self.exception2.status_code, response2.status_code)
         self.assertEqual(
             {"errors": [text_type(self.exception2)]},
-            json.loads(response2.content)
+            json.loads(response2.content.decode('utf-8'))
         )
 
         self.assertIsNone(self.a.process_exception(self.request1, self.exception0))

lms/djangoapps/instructor/tests/test_tools.py

@@ -33,7 +33,7 @@ class TestDashboardError(unittest.TestCase):
     """
     def test_response(self):
         error = tools.DashboardError(u'Oh noes!')
-        response = json.loads(error.response().content)
+        response = json.loads(error.response().content.decode('utf-8'))
         self.assertEqual(response, {'error': 'Oh noes!'})
 
 
@@ -50,7 +50,7 @@ class TestHandleDashboardError(unittest.TestCase):
             """
             raise tools.DashboardError("Oh noes!")
 
-        response = json.loads(view(None, None).content)
+        response = json.loads(view(None, None).content.decode('utf-8'))
         self.assertEqual(response, {'error': 'Oh noes!'})
 
     def test_no_error(self):