gcs: Allow compressed files to be downloaded - fixes #2658

Before this change, the go runtime would automatically decompress
compressed objects leading to length mismatches.

After this change rclone will download the compressed object which
will match in length and checksum.

backend/googlecloudstorage/googlecloudstorage.go

@@ -298,6 +298,7 @@ type Object struct {
 	bytes    int64     // Bytes in the object
 	modTime  time.Time // Modified time of the object
 	mimeType string
+	gzipped  bool // set if object has Content-Encoding: gzip
 }
 
 // ------------------------------------------------------------
@@ -899,6 +900,7 @@ func (o *Object) setMetaData(info *storage.Object) {
 	o.url = info.MediaLink
 	o.bytes = int64(info.Size)
 	o.mimeType = info.ContentType
+	o.gzipped = info.ContentEncoding == "gzip"
 
 	// Read md5sum
 	md5sumData, err := base64.StdEncoding.DecodeString(info.Md5Hash)
@@ -1026,6 +1028,15 @@ func (o *Object) Open(ctx context.Context, options ...fs.OpenOption) (in io.Read
 	}
 	req = req.WithContext(ctx) // go1.13 can use NewRequestWithContext
 	fs.FixRangeOption(options, o.bytes)
+	if o.gzipped {
+		// Allow files which are stored on the cloud storage system
+		// compressed to be downloaded without being decompressed.  Note
+		// that setting this here overrides the automatic decompression
+		// in the Transport.
+		//
+		// See: https://cloud.google.com/storage/docs/transcoding
+		req.Header.Set("Accept-Encoding", "gzip")
+	}
 	fs.OpenOptionAddHTTPHeaders(req.Header, options)
 	var res *http.Response
 	err = o.fs.pacer.Call(func() (bool, error) {