net/http: TestServerEmptyBodyRace_h1 flake

[bradfitz]

Saw this flake:

https://storage.googleapis.com/go-build-log/03165aa5/linux-amd64-race_57dd9275.log

ok      mime/quotedprintable    1.723s
ok      net 3.527s
--- FAIL: TestServerEmptyBodyRace_h1 (0.01s)
    serve_test.go:3343: handler ran 21 times; want 20
FAIL
FAIL       net/http 18.340s

[aclements]

$ findflakes -grep TestServerEmptyBodyRace_h1
First observed 6bcec09 16 Dec 20:25 2015 (125 commits ago)
Last observed  ab5d2bf 05 Jan 15:42 2016 (0 commits ago)
100% chance failure is still happening
22% failure probability (28 of 126 commits)
Likely culprits:
   22% 6bcec09 math/big: additional Montgomery cleanup
   17% 0abf443 runtime: remove incorrect TODO added in CL 16035
   13% bdd5d6e cmd/go: tiny cleanup
   10% 0c428a5 go/build: support vendor directories in Import
    8% ae9529a doc: first draft of Go 1.6 release notes
    6% 6e11f45 net/http: make Server validate Host headers
    5% c2fb457 net/url: fix RFC typo in comments
    4% 5de3ff2 runtime: accept NumGC==0 in TestMemStats
    3% 65cd1ba cmd/compile: re-vendor math/big so we use latest version in compiler
    2% 1fe3933 net/http: fix Transport race returning bodyless responses and reusing conns
No known past failures

$ greplogs -dashboard -l -md -E TestServerEmptyBodyRace_h1 | tail
2015-12-28T01:12:24-c7c7c70/linux-ppc64le-canonical
2015-12-29T21:16:59-8d5ff2e/freebsd-amd64-race
2015-12-30T06:37:08-4601776/linux-arm64-linaro
2016-01-03T20:52:07-e2093cd/freebsd-amd64-race
2016-01-04T16:33:58-956535f/freebsd-amd64-race
2016-01-04T19:23:17-28b95ed/linux-amd64-clang
2016-01-04T19:23:17-28b95ed/linux-amd64-race
2016-01-04T20:10:17-7f4443d/freebsd-amd64-race
2016-01-05T01:21:26-b80ea53/freebsd-amd64-race
2016-01-05T15:42:12-ab5d2bf/dragonfly-amd64

[gopherbot]

CL https://golang.org/cl/18282 mentions this issue.