net/http: TestServerTimeouts flake on linux-amd64-race

[josharian]

From a trybot run

--- FAIL: TestServerTimeouts (0.81s)
	serve_test.go:511: http Get #2: Get http://127.0.0.1:46301: EOF
FAIL
FAIL	net/http	10.603s

https://storage.googleapis.com/go-build-log/74eef7e1/linux-amd64-race_6374b9e8.log

[gopherbot]

CL https://golang.org/cl/38154 mentions this issue.