Fixes content type header validaton

[juanf]

Hi, I've noticed that in the tests the content-type header wasn't being validated correctly, this fixes it and adds coverage.

[juanf]

Oh @arietimmerman I see this fails because it returns 404 error instead of 400
The 400 on src/Middleware/SCIMHeaders.php is ignored:
throw new SCIMException(sprintf('The content-type header should be set to "%s"', 'application/scim+json'), 400);
This PR should fix it #68

[arietimmerman]

I've now merged in #68, I guess you'll need to rebase this branch to master to fix the failing pipeline?

[juanf]

Hi @arietimmerman thank you for your work on this package, it's super helpful.
Now some tests pass but on those failing this is what I get, I'm not sure why

Warning - The configuration file did not pass validation!
  The following problems have been detected:

  Line [11](https://github.com/limosa-io/laravel-scim-server/actions/runs/9402556040/job/25896964640?pr=70#step:5:12):
  - Element 'phpunit', attribute 'backupStaticProperties': The attribute 'backupStaticProperties' is not allowed.
  - Element 'phpunit', attribute 'cacheDirectory': The attribute 'cacheDirectory' is not allowed.

[juanf]

Hi @arietimmerman the pipeline is fixed now.