When using the syndication framework to generate parameterized feeds as described in the documentation, leaving off the parameter from the URL causes an exception to be raised in the Feed's link() function.
For example, if I'm trying to generate per-tag feeds for a blog at /feeds/tags/TAG_NAME, /feeds/tags/ will cause an AttributeError in my Feed subclass' link() function because the object passed to it is None. My proposed solution, which seems to work for me, is instead of checking the contents of url in get_feed(), we should check if the Feed class (self) has a get_object function, since from what I understand, any Feed subclass with a get_object() function is intended to generate a parameterized feed.
With my patch applied, /feeds/tags/ results in a 404, which seems like the appropriate behavior to me.
