Content-Disposition filename issue.

[xieyang784]

In lib/index.js round 232 lines:

MongoStorage.prototype.__download = function(file, res, callback) {
    var gfs, read;
    if (callback == null) {
      callback = (function() {});
    }
    gfs = Grid(this.db, mongodb);
    read = gfs.createReadStream({
      _id: file._id
    });
    res.set('Content-Disposition', "attachment; filename=\"" + file.filename + "\"");
    res.set('Content-Type', file.metadata.mimetype);
    res.set('Content-Length', file.length);
    return read.pipe(res);
  };

If file.filename contains Chinese code or other un-standard code express will throw error:

TypeError: The header content contains invalid characters
    at validateHeader (_http_outgoing.js:492:11)
    at ServerResponse.setHeader (_http_outgoing.js:496:3)  

So change:
res.set('Content-Disposition', "attachment; filename=\"" + file.filename + "\"");
to:
res.attachment(file.filename);
will correct this issue.

[xieyang784]

@jdrouet please help review it, thanks.