Fix failing tests with PHP7 / D8.4.x in the dev branch

Tested with php 7.0 and php 7.1 on Drupal 8.3.x - could not reproduce the issue. Assuming it's something in Drupal 8.4.x.

I'll get home in several hours, will take a look

Just a note while I'm still searching for the issue: was able to reproduce the issue on both php7.0 and php7.1 with latest Drupal core 8.4.x.

Got it: here's the issue which broke our tests: #2293697: EntityResource POST routes all use the confusing default: use entity types' https://www.drupal.org/link-relations/create link template if available. If I run tests with this commit in core - they fail. If I go 1 commit before - tests pass. Now going to think about the proper fix.

The issue is related to the new route added in the patch:

system.4xx:
  path: '/system/4xx'
  defaults:
   _controller: '\Drupal\system\Controller\Http4xxController:on4xx'
    _title: 'Client error'
  requirements:
    _access: 'TRUE'

So instead of json response, Drupal returns html with rendered 4xx page. Trying to find a good solution (all ideas are welcome)

So now DefaultExceptionHtmlSubscriber implements method on4xx(), which eventually returns the html page:

public function on4xx(GetResponseForExceptionEvent $event) {
  if (($exception = $event->getException()) && $exception instanceof HttpExceptionInterface) {
    $this->makeSubrequest($event, '/system/4xx', $exception->getStatusCode());
  }
}

I think that JSON API needs its own ExceptionSubscriber which provides the expected error handing.

Oh, actually DefaultExceptionSubscriber exists in JSON API. Wondering then how the error handing appears inDefaultExceptionHtmlSubscriber?

Okay, I think I know the answer to #10. We're trying to test the case when there is no Content-Type header. The request is treated as html request, and DefaultExceptionHtmlSubscriber is used instead of the one from JSON API. Since there was introduced a new method on4xx(), it simply returns its value (obviously, in html representation).

The fix for this issue turns out to be simple: instead of Content-Type header we should send Accept header to indicate that we want to get response in json format.

As a follow-up, we should create an issue to highlight in documentation how important headers when it comes to errors handing.

Fixed the same issue in several other places.

Now all tests are passing, yay! Awaiting for manual review.

Sweeeet, thanks Mateu.