Project level urls delegation and app level router

[c17r]

Checklist

• I have verified that that issue exists against the master branch of Django REST framework.
• I have searched for similar issues in both open and closed tickets and cannot find a duplicate.
• This is not a usage question. (Those should be directed to the discussion group instead.)
• This cannot be dealt with as a third party library. (We prefer new functionality to be in the form of third party libraries where possible.)
• I have reduced the issue to the simplest possible case.
• I have included a failing test as a pull request. (If you are unable to do so we can still accept the issue.)

Steps to reproduce

in project level urls.py

from django.conf.urls import url, include

urlpatterns = [
    url(r'myapp/', include('myapp.urls')),
]

in myapp/urls.py

from django.conf.urls import url, include
from rest_framework import routers

from myapp import views

router = routers.SimpleRouter()
router.register(r'', views.MyAppViewSet)

urlpatterns = [
    url(r'^', include(router.urls)),
]

Expected behavior

Calls to /myapp/ and /myapp/1/ work

Actual behavior

2 warnings:

• ?: (urls.W002) Your URL pattern '^/$' [name='myapp-list'] has a regex beginning with a '/'. Remove this slash as it is unnecessary.
• ?: (urls.W002) Your URL pattern '^/(?P[^/.]+)/$' [name='myapp-detail'] has a regex beginning with a '/'. Remove this slash as it is unnecessary.

and the urls are actually /myapp// and /myapp//1

failing test cases at #4486

[tomchristie]

Closed via #4486