handle bytes list in api middleware

In python3, the inner string is bytes, let's handle it to avoid
failure.
Also update a unit test related to response string.

Partially-Implements: blueprint magnum-python3

Change-Id: Ia85b4e4a49a1cb39a415da764343de001d61234d

magnum/api/middleware/parsable_error.py

@@ -19,6 +19,7 @@ Based on pecan.middleware.errordocument
 """
 
 import json
+import six
 
 from magnum.i18n import _
 
@@ -57,7 +58,8 @@ class ParsableErrorMiddleware(object):
 
         app_iter = self.app(environ, replacement_start_response)
         if (state['status_code'] // 100) not in (2, 3):
-            body = [json.dumps({'error_message': '\n'.join(app_iter)})]
+            body = [six.b(json.dumps({'error_message':
+                          six.b('\n').join(app_iter).decode('utf-8')}))]
             state['headers'].append(('Content-Type', 'application/json'))
             state['headers'].append(('Content-Length', str(len(body[0]))))
         else:

magnum/tests/unit/api/test_hooks.py

@@ -14,6 +14,7 @@
 # limitations under the License.
 
 import json
+import six
 
 import mock
 from oslo_config import cfg
@@ -104,8 +105,13 @@ class TestNoExceptionTracebackHook(api_base.FunctionalTest):
         # instead of'\n'.join(trace). But since RemoteError is kind of very
         # rare thing (happens due to wrong deserialization settings etc.)
         # we don't care about this garbage.
-        expected_msg = ("Remote error: %s %s"
-                        % (test_exc_type, self.MSG_WITHOUT_TRACE) + "\n[u'")
+        if six.PY2:
+            expected_msg = ("Remote error: %s %s"
+                            % (test_exc_type, self.MSG_WITHOUT_TRACE)
+                            + "\n[u'")
+        else:
+            expected_msg = ("Remote error: %s %s"
+                            % (test_exc_type, self.MSG_WITHOUT_TRACE) + "\n['")
         actual_msg = json.loads(response.json['error_message'])['faultstring']
         self.assertEqual(expected_msg, actual_msg)