If a resource is being fetched, .fetch should not return a resolved promise

It should instead return the same deferred promise as the pending AJAX
call.

Before:

```javascript
a = resource.fetch();
b = resource.fetch();
a === b; //false
a.state(); //resolved
```

After:

```javascript
a = resource.fetch();
b = resource.fetch();
a === b; //true
a.state(); //pending
```

@jish @jamesarosen @vcekov