Data race in http library

[flimzy]

WARNING: DATA RACE
Read at 0x00c420184de0 by goroutine 55:
runtime.mapaccess2_faststr()
/home/travis/.gimme/versions/go/src/runtime/hashmap_fast.go:317 +0x0
net/http.(*Request).write()
/home/travis/.gimme/versions/go/src/net/http/request.go:555 +0x59b
net/http.(*persistConn).writeLoop()
/home/travis/.gimme/versions/go/src/net/http/transport.go:1754 +0x2ee
Previous write at 0x00c420184de0 by goroutine 44:
runtime.mapassign_faststr()
/home/travis/.gimme/versions/go/src/runtime/hashmap_fast.go:600 +0x0
net/textproto.MIMEHeader.Set()
/home/travis/.gimme/versions/go/src/net/textproto/header.go:22 +0x74
net/http.Header.Set()
/home/travis/.gimme/versions/go/src/net/http/header.go:31 +0x60
net/http.(*Request).AddCookie()
/home/travis/.gimme/versions/go/src/net/http/request.go:385 +0x31b
net/http.(*Client).send()
/home/travis/.gimme/versions/go/src/net/http/client.go:170 +0x118
net/http.(*Client).Do()
/home/travis/.gimme/versions/go/src/net/http/client.go:595 +0x458
github.com/flimzy/kivik/driver/couchdb/chttp.(*Client).DoReq()
/home/travis/gopath/src/github.com/flimzy/kivik/driver/couchdb/chttp/chttp.go:182 +0x187
github.com/flimzy/kivik/driver/couchdb/chttp.(*Client).DoJSON()
/home/travis/gopath/src/github.com/flimzy/kivik/driver/couchdb/chttp/chttp.go:133 +0x9a
github.com/flimzy/kivik/driver/couchdb.(*db).Put()
/home/travis/gopath/src/github.com/flimzy/kivik/driver/couchdb/db.go:144 +0x37d
github.com/flimzy/kivik.(*DB).Put()
/home/travis/gopath/src/github.com/flimzy/kivik/db.go:70 +0xaf
github.com/flimzy/kivik/test/db.testPutAttachment.func4.1()
/home/travis/gopath/src/github.com/flimzy/kivik/test/db/putattachment.go:101 +0xeb
github.com/flimzy/kivik/test/kt.Retry()
/home/travis/gopath/src/github.com/flimzy/kivik/test/kt/kt.go:270 +0x62
github.com/flimzy/kivik/test/db.testPutAttachment.func4()
/home/travis/gopath/src/github.com/flimzy/kivik/test/db/putattachment.go:99 +0x250
github.com/flimzy/kivik/test/kt.(*Context).Run.func1()
/home/travis/gopath/src/github.com/flimzy/kivik/test/kt/kt.go:155 +0x23c
testing.tRunner()
/home/travis/.gimme/versions/go/src/testing/testing.go:746 +0x16c
Goroutine 55 (running) created at:
net/http.(*Transport).dialConn()
/home/travis/.gimme/versions/go/src/net/http/transport.go:1169 +0xbc9
net/http.(*Transport).getConn.func4()
/home/travis/.gimme/versions/go/src/net/http/transport.go:925 +0xa2
Goroutine 44 (running) created at:
testing.(*T).Run()
/home/travis/.gimme/versions/go/src/testing/testing.go:788 +0x568
github.com/flimzy/kivik/test/kt.(*Context).Run()
/home/travis/gopath/src/github.com/flimzy/kivik/test/kt/kt.go:152 +0x103
github.com/flimzy/kivik/test/db.testPutAttachment()
/home/travis/gopath/src/github.com/flimzy/kivik/test/db/putattachment.go:92 +0x5e5
github.com/flimzy/kivik/test/db.putAttachment.func1.1.2()
/home/travis/gopath/src/github.com/flimzy/kivik/test/db/putattachment.go:31 +0x95
github.com/flimzy/kivik/test/kt.(*Context).Run.func1()
/home/travis/gopath/src/github.com/flimzy/kivik/test/kt/kt.go:155 +0x23c
testing.tRunner()
/home/travis/.gimme/versions/go/src/testing/testing.go:746 +0x16c