Routes are improperly generated with a blank :path_prefix

Please see my write up here: http://www.somethingunimportant.com/2009/03/04/get-my-patch-added-to-core/
And my ticket/patch here:
http://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/2122-routing-with-a-blank-path_prefix-causes-a-bad-url-to-generate#ticket-2122-3

Brief description:
When generating routes, a nil :path_prefix is ignored, but a
blank :path_prefix ("") is treated like a string with content. In this
situation, when the strings are joined an extra slash is added to the
beginning of the path.

Sent to quickly...

This patch fixes the situation where the :path_prefix is a blank
string, and simply ignores it as if it were nil.