Check for empty router prefix; adjust URL accordingly

It's easiest to fix this issue after we have made the regex.  To try
to fix it before would require doing something different for List vs
Detail, which means we'd have to know which type of url we're
constructing before acting accordingly.

rest_framework/routers.py

@@ -83,6 +83,7 @@ class BaseRouter(object):
 
 
 class SimpleRouter(BaseRouter):
+
     routes = [
         # List route.
         Route(
@@ -258,6 +259,13 @@ class SimpleRouter(BaseRouter):
                     trailing_slash=self.trailing_slash
                 )
 
+                # If there is no prefix, the first part of the url is probably
+                #   controlled by project's urls.py and the router is in an app,
+                #   so a slash in the beginning will (A) cause Django to give
+                #   warnings and (B) generate URLS that will require using '//'.
+                if not prefix and regex[:2] == '^/':
+                    regex = '^' + regex[2:]
+
                 view = viewset.as_view(mapping, **route.initkwargs)
                 name = route.name.format(basename=basename)
                 ret.append(url(regex, view, name=name))