Update GCS storage library to support GetObject API on gzipped objects (#6506)

Fixes #6280

vendor/google.golang.org/api/gensupport/go18.go

@@ -0,0 +1,17 @@
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// +build go1.8
+
+package gensupport
+
+import (
+	"io"
+	"net/http"
+)
+
+// SetGetBody sets the GetBody field of req to f.
+func SetGetBody(req *http.Request, f func() (io.ReadCloser, error)) {
+	req.GetBody = f
+}