mailbox.MMDF wrongly adds From_ header to file

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2017-05-22.10:28:18.953>
labels = ['type-bug', 'library']
title = 'mailbox.MMDF wrongly adds From_ header to file'
updated_at = <Date 2017-05-22.10:28:18.953>
user = 'https://bugs.python.org/ralphcorderoy'

bugs.python.org fields:

activity = <Date 2017-05-22.10:28:18.953>
actor = 'ralph.corderoy'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2017-05-22.10:28:18.953>
creator = 'ralph.corderoy'
dependencies = []
files = []
hgrepos = []
issue_num = 30428
keywords = []
message_count = 1.0
messages = ['294138']
nosy_count = 1.0
nosy_names = ['ralph.corderoy']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue30428'
versions = ['Python 3.6']