Can't upload empty file to S3

[arogozhnikov]

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Hello, thx for porting object-store to python!

I'm facing this issue when trying to .put('some-key', b'') (empty object) to S3 storage, other sizes work ok:

Exception: Generic S3 error: Error performing put request data: response error "<?xml version="1.0" encoding="UTF-8"?><Error><Code>MissingContentLength</Code><Message>You must provide the Content-Length HTTP header.</Message></Error>", after 0 retries: HTTP status client error (411 Length Required) for url