Fix compatibility break between 0.4.7 and 0.4.8

If the ```data.refresh_token``` is not specified, it gets stringified to "undefined" and sent as such, which leads to an oauth error in facebook and makes it point to "http://www.facebook.com/meundefined", which in turn returns the wrong data. It is still "valid" data as far as authom is concerned though so it doesn't trigger an error.

The fix is to only specify the ```refresh_token``` if it's set instead of specifying an "undefined" ```refresh_token``` value.

For reference, here's the structured returned by facebook in these cases:

```javascript
{
 "token": "AAACKbJnOiTEBADEjSX7ZCZC7VW66lXUo9APsxxUGZABbtTnzWxHXzqMSKQyv4zWTyvShVHhfvEKeLeTCJpoATAeRbdhdZAFhAoz7HubrlAZDZD",
 "id": "144029828966095",
 "data": {
  "about": "cant say anything about UNDEFINED ME as it is not having any definition.......",
  "can_post": true,
  "category": "Community",
  "description": "A definition is a passage that explains the meaning of a term (a word, phrase or other set of symbols), or a type of thing. The term to be defined is the definiendum (plural definienda). A term may have many different senses or meanings. For each such specific sense, a definiens (plural definientia) is a cluster of words that defines it.\n\n\n\n\n\nBUT \n\n\n\n\nUndefined is an adjective referring to something which is not defined properly.\nJUST like us(WE)...........",
  "is_published": true,
  "talking_about_count": 0,
  "username": "meundefined",
  "website": "http://virajraol.wordpress.com http://virajraol.blogspot.com/",
  "were_here_count": 0,
  "id": "144029828966095",
  "name": "Undefined ME",
  "link": "http://www.facebook.com/meundefined",
  "likes": 136,
  "cover": {
   "cover_id": 411709728864769,
   "source": "https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-ash4/s720x720/309305_411709728864769_293096798_n.jpg",
   "offset_y": 0,
   "offset_x": 0
  }
 },
 "service": "fbLogin"
}
```