Rack::URLMap should match on paths that include a file extension

Requests to paths that included a file extension were not properly routing from
a URLMap. For example, the following URLMap:

Rack::URLMap.new('/widgets' => Widgets.new)

would fail to resolve 'GET /widgets.json', instead a 404 is returned. I wasn't 100% certain this behavior was correct as implemented. It would seem to make sense that file extensions should omitted when resolving the mapping.