It's been observed when the node under test has a tiny resource
configutation or package http has changed the behavior of its own
active connection pool.

For example,
http://build.golang.org/log/8912bc0944628cf1b4fe4063a77d36d19f9dd6a3

LGTM=adg
R=adg
CC=golang-codereviews
https://golang.org/cl/78640043