net/http: Transport returns chunked Response with ContentLength 0 [js/wasm]

[bradfitz]

http.Response.ContentLength should be -1 on a chunked streaming response body without a Content-Length header.

But with js/wasm, it's currently set to 0:

https://github.com/golang/go/blob/master/src/net/http/roundtrip_js.go#L133-L143

/cc @neild @johanbrandhorst

[catzkorn]

I intend to submit a fix for this

[gopherbot]

Change https://golang.org/cl/358134 mentions this issue: net/http: correct chunked encoding Content-Length for js/wasm