Remove dependency on httputil for client hijack test

Signed-off-by: Derek McGowan <derek@mcg.dev>

client/hijack_test.go

@@ -8,10 +8,10 @@ import (
 	"net/http"
 	"net/http/httptest"
 	"net/url"
+	"strings"
 	"testing"
 	"time"
 
-	"github.com/docker/docker/api/server/httputils"
 	"github.com/docker/docker/api/types"
 	"gotest.tools/v3/assert"
 	is "gotest.tools/v3/assert/cmp"
@@ -26,28 +26,33 @@ func TestTLSCloseWriter(t *testing.T) {
 		Handler: http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
 			chErr = make(chan error, 1)
 			defer close(chErr)
-			if err := httputils.ParseForm(req); err != nil {
+
+			if err := req.ParseForm(); err != nil && !strings.HasPrefix(err.Error(), "mime:") {
 				chErr <- fmt.Errorf("error parsing form: %w", err)
 				http.Error(w, err.Error(), http.StatusInternalServerError)
 				return
 			}
-			r, rw, err := httputils.HijackConnection(w)
+
+			conn, _, err := w.(http.Hijacker).Hijack()
 			if err != nil {
 				chErr <- fmt.Errorf("error hijacking connection: %w", err)
 				http.Error(w, err.Error(), http.StatusInternalServerError)
 				return
 			}
-			defer r.Close()
+			defer conn.Close()
 
-			fmt.Fprint(rw, "HTTP/1.1 101 UPGRADED\r\nContent-Type: application/vnd.docker.raw-stream\r\nConnection: Upgrade\r\nUpgrade: tcp\r\n\n")
+			// Flush the options to make sure the client sets the raw mode
+			_, _ = conn.Write([]byte{})
+
+			fmt.Fprint(conn, "HTTP/1.1 101 UPGRADED\r\nContent-Type: application/vnd.docker.raw-stream\r\nConnection: Upgrade\r\nUpgrade: tcp\r\n\n")
 
 			buf := make([]byte, 5)
-			_, err = r.Read(buf)
+			_, err = conn.Read(buf)
 			if err != nil {
 				chErr <- fmt.Errorf("error reading from client: %w", err)
 				return
 			}
-			_, err = rw.Write(buf)
+			_, err = conn.Write(buf)
 			if err != nil {
 				chErr <- fmt.Errorf("error writing to client: %w", err)
 				return