fix another panic where CloseNotify was called from wrong goroutine

    panic: net/http: CloseNotify called after ServeHTTP finished

    goroutine 180 [running]:
    net/http.(*response).CloseNotify(0xc8220684e0, 0x0)
        /home/r/go/src/net/http/server.go:1535 +0x9d
    github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/prometheus/client_golang/
        /home/r/src/github.com/ipfs/go-ipfs/Godeps/_workspace/src/github.com/prome
    github.com/ipfs/go-ipfs/core/corehttp.(*gatewayHandler).getOrHeadHandler.func1(0x7
        /home/r/src/github.com/ipfs/go-ipfs/core/corehttp/gateway_handler.go:119 +
    created by github.com/ipfs/go-ipfs/core/corehttp.(*gatewayHandler).getOrHeadHandle
        /home/r/src/github.com/ipfs/go-ipfs/core/corehttp/gateway_handler.go:123 +

License: MIT
Signed-off-by: Robert Carlsen <rwcarlsen@gmail.com>

core/corehttp/gateway_handler.go

@@ -114,9 +114,10 @@ func (i *gatewayHandler) getOrHeadHandler(w http.ResponseWriter, r *http.Request
 	defer cancel()
 
 	if cn, ok := w.(http.CloseNotifier); ok {
+		clientGone := cn.CloseNotify()
 		go func() {
 			select {
-			case <-cn.CloseNotify():
+			case <-clientGone:
 			case <-ctx.Done():
 			}
 			cancel()