Compare headers to correct definition of str

Author: jpmelos

Diff for: src/future/backports/email/base64mime.py

@@ -28,6 +28,7 @@
 from __future__ import absolute_import
 from future.builtins import range
 from future.builtins import bytes
+from future.builtins import str
 
 __all__ = [
     'body_decode',