smtplib send_message should add Date header if it is missing, per RFC5322

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2016-12-05.15:48:15.178>
labels = ['3.7', 'easy', 'type-bug', 'library']
title = 'smtplib send_message should add Date header if it is missing, per RFC5322'
updated_at = <Date 2018-09-08.23:03:27.435>
user = 'https://bugs.python.org/HenningvonBargen'

bugs.python.org fields:

activity = <Date 2018-09-08.23:03:27.435>
actor = '_savage'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-12-05.15:48:15.178>
creator = 'Henning.von.Bargen'
dependencies = []
files = ['45929', '45958', '45959', '45969', '46001', '46002', '46459', '46460']
hgrepos = []
issue_num = 28879
keywords = ['patch', 'easy']
message_count = 32.0
messages = ['282425', '282429', '282549', '282553', '282558', '283275', '283313', '283347', '283361', '283362', '283412', '283431', '283436', '283572', '283574', '283593', '283604', '283645', '283647', '283847', '283848', '283849', '285686', '285694', '285697', '286037', '286504', '286505', '288046', '297496', '297528', '324865']
nosy_count = 5.0
nosy_names = ['r.david.murray', 'maciej.szulik', 'Henning.von.Bargen', '_savage', 'Eric Lafontaine']
pr_nums = ['2655', '5176']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28879'
versions = ['Python 3.6', 'Python 3.7']