writing nested multipart messages does not work correctly

[terencehonles]

Long story short

Write the following multipart message:

from aiohttp.multipart import MultipartWriter
with MultipartWriter('mixed') as root:
  with MultipartWriter('mixed') as subwriter1:
    subwriter1.append('first message')
  root.append(subwriter1)

  with MultipartWriter('mixed') as subwriter2:
    subwriter2.append('second message')
  root.append(subwriter2)

print(b''.join(root.serialize()).decode('ascii'))

Expected behaviour

--0d20b463316a462d839e93130150f5ef
Content-Type: multipart/mixed; boundary="05af9536a8374279a8bccc2ea099c82b"

--05af9536a8374279a8bccc2ea099c82b
Content-Type: text/plain; charset=utf-8
Content-Length: 13

first message
--05af9536a8374279a8bccc2ea099c82b--

--0d20b463316a462d839e93130150f5ef
Content-Type: multipart/mixed; boundary="1d4fdf1ddf8e46019da125c22c247649"

--1d4fdf1ddf8e46019da125c22c247649
Content-Type: text/plain; charset=utf-8
Content-Length: 14

second message
--1d4fdf1ddf8e46019da125c22c247649--

--0d20b463316a462d839e93130150f5ef--

Actual behaviour

--d3b608e9626a4f3cb2329fa01ebb1fce
Content-Type: application/octet-stream

--9310b9861e56425daf7ef0773fd2ae54
Content-Type: text/plain; charset=utf-8
Content-Length: 13

first message
--9310b9861e56425daf7ef0773fd2ae54--

--d3b608e9626a4f3cb2329fa01ebb1fce
Content-Type: application/octet-stream

--6fa9c511ee2d4a36bec615b15da6f1b2
Content-Type: text/plain; charset=utf-8
Content-Length: 14

second message
--6fa9c511ee2d4a36bec615b15da6f1b2--

--d3b608e9626a4f3cb2329fa01ebb1fce--

Steps to reproduce

Writer a nested multipart message as shown in the documentation.

"Fix": append(subwriter, headers=subwriter.headers)

I'll try to commit a patch, but just reporting the issue right now

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