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

Fixes #6280

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

@@ -5,6 +5,8 @@
 package gensupport
 
 import (
+	"encoding/json"
+	"errors"
 	"net/http"
 
 	"golang.org/x/net/context"
@@ -32,6 +34,11 @@ func RegisterHook(h Hook) {
 // If ctx is non-nil, it calls all hooks, then sends the request with
 // ctxhttp.Do, then calls any functions returned by the hooks in reverse order.
 func SendRequest(ctx context.Context, client *http.Client, req *http.Request) (*http.Response, error) {
+	// Disallow Accept-Encoding because it interferes with the automatic gzip handling
+	// done by the default http.Transport. See https://github.com/google/google-api-go-client/issues/219.
+	if _, ok := req.Header["Accept-Encoding"]; ok {
+		return nil, errors.New("google api: custom Accept-Encoding headers not allowed")
+	}
 	if ctx == nil {
 		return client.Do(req)
 	}
@@ -53,3 +60,12 @@ func SendRequest(ctx context.Context, client *http.Client, req *http.Request) (*
 	}
 	return resp, err
 }
+
+// DecodeResponse decodes the body of res into target. If there is no body,
+// target is unchanged.
+func DecodeResponse(target interface{}, res *http.Response) error {
+	if res.StatusCode == http.StatusNoContent {
+		return nil
+	}
+	return json.NewDecoder(res.Body).Decode(target)
+}