Update Accept.php

[mgralikowski]

Fixes:

preg_match(): Passing null to parameter #2 ($subject) of type string is deprecated in /var/www/html/vendor/api-ecosystem-for-laravel/dingo-api/src/Http/Parser/Accept.php on line 70  

Fix #37

[mgralikowski]

Ok, I noticed another PR that resolves this issue too, but minimally in a different way :)
Mine uses more Laravel-way as uses a native functionality of the "header" method.

#39

[mgralikowski]

@specialtactics could you review my PR? It's trivial ;)

[specialtactics]

Thank you @mgralikowski tagged as [v4.1.2]