Return valid feedback from ORA stub

so that the logs will be much cleaner.
See test_root/log/bok_choy_lms.log

common/test/acceptance/tests/test_ora.py

@@ -204,7 +204,7 @@ class AIAssessmentTest(OpenResponseTest):
 
     XQUEUE_GRADE_RESPONSE = {
         'score': 1,
-        'feedback': {"spelling": "Ok.", "grammar": "Ok.", "markup_text": "NA"},
+        'feedback': json.dumps({"spelling": "Ok.", "grammar": "Ok.", "markup_text": "NA"}),
         'grader_type': 'BC',
         'success': True,
         'grader_id': 1,
@@ -249,7 +249,7 @@ class InstructorAssessmentTest(OpenResponseTest):
 
     XQUEUE_GRADE_RESPONSE = {
         'score': 1,
-        'feedback': {"feedback": "Good job!"},
+        'feedback': json.dumps({"feedback": "Good job!"}),
         'grader_type': 'IN',
         'success': True,
         'grader_id': 1,