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.