NegotiationMiddleware calls $request->setRequestFormat('html') when there is no _format request parameter, but shouldn't

Problem/Motivation

This core bug blocks this JSON API contrib module issue: #2831137: [PP-1] Assume the use of the 'api_json' format for routes managed by JSON API.

Observation 1

Currently, the fallback for \Drupal\Core\StackMiddleware\NegotiationMiddleware::getContentType() (i.e. if no ?_format= querystring is specified) is this:

    // Do HTML last so that it always wins.
    return 'html';

And \Drupal\Core\StackMiddleware\NegotiationMiddleware::handle() does this:

    // Determine the request format using the negotiator.
    if ($requested_format = $this->getContentType($request)) {
      $request->setRequestFormat($requested_format);
    }

The end result is that $requested_format is never empty. It always is set to 'html'.

Observation 2

Now let's look at \Symfony\Component\HttpFoundation\Request::getRequestFormat(), which is what the router (and controllers) use to determine the request format

    /**
     * Gets the request format.
     *
     * Here is the process to determine the format:
     *
     *  * format defined by the user (with setRequestFormat())
     *  * _format request parameter
     *  * $default
     *
     * @param string $default The default format
     *
     * @return string The request format
     */
    public function getRequestFormat($default = 'html')
    {
        if (null === $this->format) {
            $this->format = $this->get('_format', $default);
        }

        return $this->format;
    }

Result

The result is that Request::getRequestFormat()'s $default parameter is broken in Drupal, because NegotiationMiddlewarealways sets it to 'html'. Even when there is no _format request parameter.

In other words: NegotiationMiddleware breaks the Symfony Request API.

Proposed resolution

Only let NegotiationMiddleware call Request::setFormat() when there actually is a _format request parameter.

Remaining tasks

TBD

User interface changes

None.

API changes

None.

Data model changes

None.