Fix sync xhr in Firefox

[mnaoumov]

Fixes #46

[jpillora]

Thanks for the PR, though is sync+withCreds xhr allowed in other browsers?

[mnaoumov]

@jpillora According to W3C spec it is not allowed https://www.w3.org/TR/XMLHttpRequest/#the-withcredentials-attribute

When set: throws an "InvalidAccessError" exception if the synchronous flag is set and the JavaScript global environment is a document environment.

In reality only Firefox enforces this rule so far

UPD: I think there is no need to support the functionality that is against the spec even if other browsers are not too strict to the spec yet. So I would merge my pull request without any doubts.

But of course you might consider to add browser detection to cover the bug

[garrylachman]

Same in chrome now...
Synchronous XMLHttpRequest on the main thread is deprecated because of its detrimental effects to the end user's experience. For more help, check https://xhr.spec.whatwg.org/.Setting 'XMLHttpRequest.withCredentials' for synchronous requests is deprecated.

xhook.before is not working

[morsdyce]

Hi There @mnaoumov

I've looked at this PR and could not get a failing test case to work.
In your current PR the provided test doesn't actually check anything.

I've modified the test to check that the request does finish and it passes even with a sync XHR.

Here is the code for that test:

it('sync XHR should not fail', function(done) {
      var xhr = new XMLHttpRequest();
      xhr.open('GET', '../example/example1.txt', false);
      xhr.onload = function() {
        expect(xhr.responseText).to.contain('the first text file');
        done();
      };

      xhr.send();
    });

Is this still a problem for you? Could you provide a small example where this isn't working?

Thanks!