[HttpFoundation] Optimize ServerBag::getHeaders()
[HttpFoundation] Optimize ServerBag::getHeaders()

isset() vs in_array makes it take half the time (1ms/req here)
substr() does not have to scan the whole string so it's a wee bit faster

symfony/HttpFoundation,ygerasimov/HttpFoundation,nichdu/HttpFoundation,symfony/http-foundation,oswaldderiemaecker/HttpFoundation,mneuhaus/HttpFoundation