Added `param` parameter to `res.format` that checks for the `req.params.format` variable in order to determine the requested content-type.

Works exactly the same when no `param` is set. When set to `true`, tries to override the requested content-type by using `req.params.format`. When set to a string, e.g. either `json` or `application/json`, it tries to override the requested content-type by using that string.