Apache always sets Content-Type: text/html, even for DELETE requests

Problem/Motivation

While working on #2737719: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method, I noticed that responses to DELETE requests correctly have status 204, correctly have no body, but incorrectly have a Content-Type header, and it's even set tot he completely nonsensical text/html; charset=UTF-8. See https://tools.ietf.org/html/rfc2616#section-10.2.5 and #2737719-94: EntityResource: Provide comprehensive test coverage: for every entity type, every format, every method.

The root cause: \Symfony\Component\HttpFoundation\Response::prepare():

            // Fix Content-Type
            $charset = $this->charset ?: 'UTF-8';
            if (!$headers->has('Content-Type')) {
                $headers->set('Content-Type', 'text/html; charset='.$charset);
            } elseif (0 === stripos($headers->get('Content-Type'), 'text/') && false === stripos($headers->get('Content-Type'), 'charset')) {
                // add the charset
                $headers->set('Content-Type', $headers->get('Content-Type').'; charset='.$charset);
            }

Nope, this was fixed a long time ago: https://github.com/symfony/symfony/issues/12744 + https://github.com/symfony/symfony/commit/75abd1a451d97f5075128ea10b2598....

The real root cause is PHP: https://bugs.php.net/bug.php?id=70189. This bug exists even in PHP 7.0 (at least in 7.0.12). The fix was committed in March 2016, but it either didn't make it to PHP 7, or it didn't actually solve the problem.

I can reproduce this both with the CLI (when running PHPUnit), and when accessing Drupal via Apache (when performing actual DELETE requests to an actual Drupal instance).

Proposed resolution

Fix the logic.
Wait for PHP to be fixed, then make the tests more strict.

Remaining tasks

TBD

User interface changes

None.

API changes

None.

Data model changes

None.