Fix page composer block validation

[gremo]

Subject

I am targeting this branch, because {reason}.

Closes #1318.

Changelog

### Fixed 
- page composer block validation

Explanation (copy-past from the issue #1318 )

What's happening then? Well, this commit changed the way Ajax requests are threated. This is the related PR.

To put it simple:

1. Page composer send requests using Ajax without any Accept header, that is */*
2. That commit changed the Ajax request handling, as */* is now inside the getAcceptableContentTypes() array:

private function handleXmlHttpRequestErrorResponse(Request $request, FormInterface $form): ?JsonResponse
{
    if (empty(array_intersect(['application/json', '*/*'], $request->getAcceptableContentTypes()))) {
        // Do not handle the XmlHttpRequest
        return null;
    }

   // Return a JSON response
}

Before that commit, the same methods looks like:

private function handleXmlHttpRequestErrorResponse(Request $request, FormInterface $form): ?JsonResponse
{
    if (!\in_array('application/json', $request->getAcceptableContentTypes(), true)) {
        // Do not handle the XmlHttpRequest
        return null;
    }

     // Return a JSON response
}

3. As a result, a JSON response is returned but SonataPageBundle can only handle HTML responses

Easy fix is adding an Accept header in assets/src/js/composer.js, inside the props of $.ajax:

headers: {
  Accept: 'text/html, application/xhtml+xml;'
},

.

[VincentLanglet]

I just find out that the same code exist here: https://github.com/sonata-project/SonataPageBundle/blob/3.x/src/Resources/public/sonata-page.back.js#L39

I don't know why it's duplicated.

[gremo]

@VincentLanglet you mean I have to change also that file? Isn't automatically built upon modification of composer.js?

[VincentLanglet]

@VincentLanglet you mean I have to change also that file? Isn't automatically built upon modification of composer.js?

I really don't know how this works, but nothing is updated when running yarn build.

so I updated all this files manually in #1323.

[gremo]

Thank you @VincentLanglet 🙏 are you planning a new release soon? So I can test it right now!