fixed typo in client.js

Author: charlietilt

lib/client.js

@@ -278,7 +278,7 @@ Client.prototype.upload = function(src, dest, callback) {
 Client.prototype.download = function(src, dest, callback) {
   var self = this;
   var transferInterval, transferIntervalId, lastBytes, totalBytes;
-  transferInterval = this.options.transferInterval || 5000;
+  transferInterval = this._options.transferInterval || 5000;
   lastBytes = 0;
 
   self.sftp(function(err,sftp){