fix for append with empty flags array

Calling append with options set to:
````
options: {"flags":[],"date":"2011-10-17T14:36:48.000Z"}
````
will cause an error:
````
Error removing attachments during undefined: { [Error: Invalid Arguments: Unable to parse flag \]
  level: 'protocol',
  code: undefined,
  request: 'APPEND "myfolder" (\\) "17-Oct-2011 07:36:48 --700" {2538}' }
````
The attached pull request fixes this.