Message parsing fails where it has incompele headers

[reb00ter]

BPO	32634
Nosy	@warsaw, @bitdancer, @vadmium, @reb00ter
PRs	bpo-32634: fix write in email/generator.py #5267
Superseder	bpo-27321: Email parser creates a message object that can't be flattened

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-01-23.17:34:20.752>
created_at = <Date 2018-01-23.08:31:16.269>
labels = ['type-bug', 'expert-email']
title = 'Message parsing fails where it has incompele headers'
updated_at = <Date 2018-01-23.17:34:20.751>
user = 'https://github.com/reb00ter'

bugs.python.org fields:

activity = <Date 2018-01-23.17:34:20.751>
actor = 'r.david.murray'
assignee = 'none'
closed = True
closed_date = <Date 2018-01-23.17:34:20.752>
closer = 'r.david.murray'
components = ['email']
creation = <Date 2018-01-23.08:31:16.269>
creator = 'reb00ter'
dependencies = []
files = []
hgrepos = []
issue_num = 32634
keywords = ['patch']
message_count = 3.0
messages = ['310485', '310492', '310515']
nosy_count = 4.0
nosy_names = ['barry', 'r.david.murray', 'martin.panter', 'reb00ter']
pr_nums = ['5267']
priority = 'normal'
resolution = 'duplicate'
stage = 'resolved'
status = 'closed'
superseder = '27321'
type = 'behavior'
url = 'https://bugs.python.org/issue32634'
versions = ['Python 3.4', 'Python 3.5', 'Python 3.6']

[reb00ter]

If email message dont have a content-transfer-encoding or content-type header, function write in generator.py crashes

[vadmium]

Looks like a dupe of bpo-27321

[bitdancer]

Yes, it is.