Fix removal of url_path on @detail_route and @list_route. Fix # #2583

SimpleRouter.get_routes was popping out the url_path kwarg from
list_route and detail_route decorators. This was causing troubles
when the route was re-used, for example if the viewset was
inherited.

rest_framework/routers.py

@@ -171,9 +171,9 @@ class SimpleRouter(BaseRouter):
                 # Dynamic detail routes (@detail_route decorator)
                 for httpmethods, methodname in detail_routes:
                     method_kwargs = getattr(viewset, methodname).kwargs
-                    url_path = method_kwargs.pop("url_path", None) or methodname
                     initkwargs = route.initkwargs.copy()
                     initkwargs.update(method_kwargs)
+                    url_path = initkwargs.pop("url_path", None) or methodname
                     ret.append(Route(
                         url=replace_methodname(route.url, url_path),
                         mapping=dict((httpmethod, methodname) for httpmethod in httpmethods),
@@ -184,9 +184,9 @@ class SimpleRouter(BaseRouter):
                 # Dynamic list routes (@list_route decorator)
                 for httpmethods, methodname in list_routes:
                     method_kwargs = getattr(viewset, methodname).kwargs
-                    url_path = method_kwargs.pop("url_path", None) or methodname
                     initkwargs = route.initkwargs.copy()
                     initkwargs.update(method_kwargs)
+                    url_path = initkwargs.pop("url_path", None) or methodname
                     ret.append(Route(
                         url=replace_methodname(route.url, url_path),
                         mapping=dict((httpmethod, methodname) for httpmethod in httpmethods),