Add Content-Length header to HTTP response in FilesRouter

[vbsteven]

Files served by the old Parse.com service always have a Content-Length header attached. This patch adds the same header to files served by the parse-server.

[TylerBrock]

Thanks for contributing and making the project a great one!

[gowridev]

@TylerBrock @flovilmart, trying to read the downloaded file chunk size (while downloading a file from parse file URL), we are getting -1 when trying to access "totalBytesExpectedToWrite" function (in objective C) . I think, there some issue with Content-Length header may be not getting updated (referring to FilesRouter.js).
Could please guide us in this regard.

ios code is given below

(void)URLSession:(NSURLSession )session downloadTask:(NSURLSessionDownloadTask )downloadTask didWriteData:(int64_t)bytesWritten totalBytesWritten:(int64_t)totalBytesWritten totalBytesExpectedToWrite:(int64_t)totalBytesExpectedToWrite