net/http: TestTransportBodyReadError flake

[ianlancetaylor]

Seen from trybot:

https://storage.googleapis.com/go-build-log/dba99eff/openbsd-amd64-60_9893d709.log

--- FAIL: TestTransportBodyReadError (0.01s)
	client_test.go:1821: Got error: net/http: HTTP/1.x transport connection broken: some body read error; want Request.Body read error: some body read error
FAIL
FAIL	net/http	4.860s

[bradfitz]

(Others: feel free to debug this before I get to it.)

[gopherbot]

CL https://golang.org/cl/37495 mentions this issue.