Fix failing test in the dev branch

I'll check if I able to resolve this issue quickly.

Observation: on the local php 7.1 it passes without any issues.

New observation: it's not related to php version, it's related to Drupal core version. 8.3.2 didn't have this issue, while latest 8.3.x. is causing it.

The failure happens in JsonApiFunctionalTest test, method testRead(). This code causes the issue:

    $collection_output = Json::decode($this->drupalGet('/jsonapi/node/article', [
      'query' => ['page' => ['offset' => 3]],
    ]));
    ...
    $this->assertContains('page[offset]=53', $collection_output['links']['next']);

In Drupal 8.3.2, $collection_output['links']['next'] looks like this:

http://hostname/jsonapi/node/article?page[offset]=53&page[limit]=50

In Drupal 8.3.x it looks like this:

http://hostname/jsonapi/node/article?page%5Boffset%5D=53&page%5Blimit%5D=50

Looks like something is broken in serialization. Keep digging.

I've found the issue which broke our tests, it's this one: #1827854: W3C HTML5 Validation error with PHP array-based query-strings built with url() containing [ ] characters (eg views filters).
So it's not a bug, it's a feature and our tests have to comply with this.

Attaching patch which resolves the issue.

Great, thanks Mateu for the fast turnaround