net/http: Transport race condition by Content-Length == 0 response [1.14 backport]

[gopherbot]

@networkimprov requested issue #41600 to be considered for backport to the next 1.14 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.

[dmitshur]

Closing because Go 1.16 is out, and there will not be a Go 1.14.16 minor release per release policy.