test.testWSGI: Make "request" return response headers as a dict.

Concatenate potential homonymous headers following specification.

caucase/test.py

@@ -1353,7 +1353,12 @@ class CaucaseTest(unittest.TestCase):
       (status, header_list), = start_response_list
       # pylint: enable=unbalanced-tuple-unpacking
       status, reason = status.split(' ', 1)
-      return int(status), reason, header_list, ''.join(body)
+      header_dict = {}
+      for key, value in header_list:
+        if key in header_dict: # pragma: no cover
+          value = header_dict[key] + ',' + value
+        header_dict[key] = value
+      return int(status), reason, header_dict, ''.join(body)
     UNAUTHORISED_STATUS = 401
 
     self.assertEqual(request({