Fix common download problem

python/paddle/dataset/common.py

@@ -19,6 +19,7 @@ import hashlib
 import os
 import errno
 import shutil
+import six
 import sys
 import importlib
 import paddle.dataset
@@ -94,6 +95,8 @@ def download(url, module_name, md5sum, save_name=None):
                 dl = 0
                 total_length = int(total_length)
                 for data in r.iter_content(chunk_size=4096):
+                    if six.PY2:
+                        data = six.b(data)
                     dl += len(data)
                     f.write(data)
                     done = int(50 * dl / total_length)