use original data object when the mock data is empty

Hello,


I'm using mockjax on my project. This is very useful library!

BTW, I think mockjax should pass through the original data object when the mock data is empty.

```javasript

// Set proxy only
$.mockjax({
    url: "/foo",
    proxy: "/bar"
});

// This will call /bar?test=test
$.ajax({
   url: "/foo",
   type: 'GET',
   data: {
      test: "test"
   }
});
```

What do you think?

Thanks,
westzaki