Fix numTokens call.

This was calling the api with a blank src parameter, which caused it to return a 400 (presumably because it couldn't parse the POST data).  I reproduced this with curl:

```
$ curl -d "src&username=foo&key=bar" "http://pdfcrowd.com/api/user/foo/tokens/"
Missing username or/and the api key
```
```
$ curl -d "username=foo&key=bar" "http://pdfcrowd.com/api/user/foo/tokens/"
84
```
This update omits the src param if the value is blank.