MultipartWriter does not support binary content encoding

[AleksandrIakhnev]

Long story short

There is no support of binary encoding in MultipartWriter
Some services require binary encoding. Seems that easy to implement with just returning unprocessed chunks.

Expected behaviour

Supported binary encoding

Actual behaviour

Not implemented encoding
https://github.com/KeepSafe/aiohttp/blob/master/aiohttp/multipart.py#L858

Steps to reproduce

Specify header 'Content-Transfer-Encoding' to 'binary' in MultiPartWriter

[kxepal]

Good catch. Please check #1169

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