Unirest is running asynchronously

Hello,

I have noticed that unirest 0.6.0 is running asynchronously.
The code reproducing is
```
unirest
.post ("http://localhost/serverurl")
.headers ({ 'Content-Type': 'application/x-www-form-urlencoded;' })
.end ((x) => console.log("OK " + JSON.stringify(x)));
console.log("End");
```

The excepted output is
````
OK { json data }
End
````

But got
````
End
OK { json data }
````

Thanks.

The issue names no source file or test. Start with the posted unirest.post(...).end(...) reproducer and verify the callback ordering; resolution would need to define whether .end should remain asynchronous or produce the requested ordering, then add regression coverage.

Written by the indexing model from the issue text.