multipart/related header causes false positive StartBoundaryNotFoundDefect and MultipartInvariantViolationDefect

GitHub fields:

assignee = None
closed_at = None
created_at = <Date 2019-03-07.15:31:17.399>
labels = ['3.7', 'type-bug', 'library']
title = 'multipart/related header causes false positive StartBoundaryNotFoundDefect and MultipartInvariantViolationDefect'
updated_at = <Date 2019-09-04.15:58:20.261>
user = 'https://github.com/grische'

bugs.python.org fields:

activity = <Date 2019-09-04.15:58:20.261>
actor = 'ned.deily'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2019-03-07.15:31:17.399>
creator = 'cschmidbauer'
dependencies = []
files = []
hgrepos = ['381']
issue_num = 36226
keywords = ['patch']
message_count = 5.0
messages = ['337395', '337401', '337444', '337661', '351114']
nosy_count = 6.0
nosy_names = ['barry', 'r.david.murray', 'martin.panter', 'maxking', 'tzickel', 'cschmidbauer']
pr_nums = ['12214']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue36226'
versions = ['Python 3.7']