net/http: Transport race condition by Content-Length == 0 response [1.15 backport]

[gopherbot]

@networkimprov requested issue #41600 (fixed by 212d385 and 854a2f8 in Go 1.16) to be considered for backport to the next 1.15 minor release.

@gopherbot please backport

The relevant CL is https://golang.org/cl/257818 - net/http: ignore connection closes once done with the connection

[toothrot]

This is a serious issue with no workaround. Approved.

[gopherbot]

Change https://golang.org/cl/297909 mentions this issue: [release-branch.go1.15] net/http: ignore connection closes once done with the connection

[gopherbot]

Change https://golang.org/cl/297910 mentions this issue: [release-branch.go1.15] net/http: add connections back that haven't been canceled

[dmitshur]

I've mailed CL 297909 and CL 297910 that will resolve this issue.