Uploading: Provide a way to not set Content-Length

[yutakahirano]

The current spec says:

1. Let contentLengthValue be null.
2. If httpRequest's body is null and httpRequest's method is POST or PUT, then set contentLengthValue to 0.
3. If httpRequest's body is non-null, set contentLengthValue to httpRequest's body's total bytes, utf-8 encoded.
4. If contentLengthValue is non-null, append Content-Length/contentLengthValue to httpRequest's header list.

We should not append a Content-Length header when uploading a Request made with a ReadableStream.

[annevk]

We should probably annotate body in some way. Either it's known-length (everything we do today) or unknown (streams).

[wanderview]

We should probably annotate body in some way. Either it's known-length (everything we do today) or unknown (streams).

FWIW, this matches exactly what we have to do at the implementation level. So sounds good to me.