`recall` should be `path_parameters`, also make it required

"recall" is a terrible name.  This variable contains the parameters that
we got from the path (e.g. for "/posts/1" it has :controller => "posts",
:id => "1").  Since it contains the parameters we got from the path,
"path_parameters" is a better name.  We always pass path_parameters to
`generate`, so lets make it required.