fix attach

Signed-off-by: Victor Vieux <vieux@docker.com>

api/api.go

@@ -259,11 +259,17 @@ func proxyHijack(c *context, w http.ResponseWriter, r *http.Request) {
 		errc := make(chan error, 2)
 		cp := func(dst io.Writer, src io.Reader) {
 			_, err := io.Copy(dst, src)
+			if conn, ok := dst.(interface {
+				CloseWrite() error
+			}); ok {
+				conn.CloseWrite()
+			}
 			errc <- err
 		}
 		go cp(d, nc)
 		go cp(nc, d)
 		<-errc
+		<-errc
 	}
 }