to use format from request if it was set

This is about last question in comment https://github.com/FriendsOfSymfony/FOSRestBundle/issues/310#issuecomment-26479952 and a few issues more.

It was very helpful sometimes to specify format in forward method. But now FormatListener ignores format in request. I offer first to check if the format was manualy set for request (for example, as an attribute in forward method), and use it, rather than trying to guess it.
its common practice in Symfony2 to use ``null ===``