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

[roperzh]

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.

Currently the xml request for this:

freshbooks.call("staff.current", {}, function(err, response) {
  console.log(response);
});

Is formatted like this:

<?xml version='1.0' encoding='utf-8'?>
<request />

And the API returns:

{ 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.