fixed a bug in Response content-type auto-detection
fixed a bug in Response content-type auto-detection

Without this patch, if you call __toString() on a Response,
the content-type auto-detection would never be trigerred
as __toString() changes the default content-type.

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