populate status for readystate 2 in more cases

[euank]

I've added a test, but unfortunately the test was passing before this change so it doesn't exhibit the true problem.

The issue I've run into is using xhook via xdomain with the aws-sdk.

Specifically, if I have the following code in the browser:

<script src="my-copy-of-xdomain-built-with-new-xhook/xdomain.js" debug="true" slave="https://s3.amazonaws.com/my-bucket/proxy.html"></script>

var creds = new AWS.Credentials({
  accessKeyId: 'akid',
  secretAccessKey: 'asecret',
});
AWS.config.update({region: 'us-east-1', credentials: creds, logger: console, s3ForcePathStyle: true});
var s3 = new AWS.S3();

s3.listBuckets().promise()
.then(data => console.log(data))
.catch(err => console.log(err));

Without this change, that code results in an error because the readyState == 3 change has not yet set 'status'.

With this change, the above code seems to work correctly.

I think that the main difference between the test environment and the AWS request is that the AWS request is chunked, so the headers really are sent before the body... I couldn't figure out a way to reproduce the error in the mocha test environment.

If you're interested, the aws-sdk code making the request which fails without this change is here.