Backups are stored with user configured encoding

I noticed in https://github.com/Microsoft/vscode/blob/master/src/vs/workbench/services/backup/node/backupFileService.ts#L73 you are not passing any encoding. This means backups are potentially stored with a user choice of encoding which is not good because we should assume UTF-8 when reading backups.

I suggest to be consistent and use UTF-8 for reading and writing of all backups.