Allow to send an empty object as a json parameter in Freshbooks#call

Hello,

First of all thanks for this awesome & super-simple library.

Actually, some of the endpoints in the freshbooks API doesn't require any parameters, for example [staff.current](http://developers.freshbooks.com/docs/staff/#staff.current).

Currently the xml request for this:

```javascript
freshbooks.call("staff.current", {}, function(err, response) {
  console.log(response);
});
```

Is formatted like this:

```xml
<?xml version='1.0' encoding='utf-8'?>
<request />
```

And the API returns:

```javascript
{ response: { error: 'Your XML is not formatted correctly.', code: '40010' } }
```

With this simple change you can pass empty objects or even `null` as a second argument to `Freshbooks#call`.