On a proprietary test ThreadSanitizer says:

WARNING: DATA RACE at 0x00004020eb40
Write by goroutine 32:
  net/http.ReadResponse()
      src/pkg/net/http/response.go:110 +0x13c
  net/http.(*persistConn).readLoop()
      src/pkg/net/http/transport.go:564 +0x3f8
Previous read by goroutine 33:
  net/http.(*Request).write()
      src/pkg/net/http/request.go:321 +0xb81
  net/http.(*persistConn).writeLoop()
      src/pkg/net/http/transport.go:641 +0x231
Goroutine 32 (running) created at:
  net/http.(*Transport).getConn()
      src/pkg/net/http/transport.go:390 +0x769
  net/http.(*Transport).RoundTrip()
      src/pkg/net/http/transport.go:156 +0x31e
  net/http.send()
      src/pkg/net/http/client.go:141 +0x4e1
  net/http.(*Client).doFollowingRedirects()
      src/pkg/net/http/client.go:254 +0x8d8
  net/http.(*Client).Get()
      src/pkg/net/http/client.go:201 +0xc1
  net/http.Get()
      src/pkg/net/http/client.go:178 +0x55
  ...
  testing.tRunner()
      src/pkg/testing/testing.go:292 +0xb6
Goroutine 33 (running) created at:
  net/http.(*Transport).getConn()
      src/pkg/net/http/transport.go:391 +0x780
  net/http.(*Transport).RoundTrip()
      src/pkg/net/http/transport.go:156 +0x31e
  net/http.send()
      src/pkg/net/http/client.go:141 +0x4e1
  net/http.(*Client).doFollowingRedirects()
      src/pkg/net/http/client.go:254 +0x8d8
  net/http.(*Client).Get()
      src/pkg/net/http/client.go:201 +0xc1
  net/http.Get()
      src/pkg/net/http/client.go:178 +0x55
  ...
  testing.tRunner()
      src/pkg/testing/testing.go:292 +0xb6

During invocation of RoundTrip() string field Request.Method is concurrently read and
mutated.