fix openapi // path prefix

[n2ygk]

Description

Fixes // prefix for a path that has a common prefix. Prior to this fix, urls with a common prefix
would result in a doubled /. For example:

router = routers.DefaultRouter()
router.register(r'courses', views.CourseViewSet)

urlpatterns = [
    path('v1/', include(router.urls)),

generates this schema:

paths:
  //courses/:
    get:

Here's reproducing it showing that the assumption of trailing / being returned (as documented) is incorrect:

django-rest-framework/rest_framework/schemas/generators.py

Lines 239 to 253 in 3e210ae

	def determine_path_prefix(self, paths):
	"""
	Given a list of all paths, return the common prefix which should be
	discounted when generating a schema structure.
	This will be the longest common string that does not include that last
	component of the URL, or the last component before a path parameter.
	For example:
	/api/v1/users/
	/api/v1/users/{pk}/
	The path prefix is '/api/v1/'
	"""

Python 3.6.6 (default, Jul 27 2018, 14:31:43) 
[GCC 4.2.1 Compatible Apple LLVM 9.1.0 (clang-902.0.39.2)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
(InteractiveConsole)
>>> from rest_framework.schemas.openapi import SchemaGenerator
>>> s=SchemaGenerator()
>>> paths=['/api/v1/users/', '/api/v1/users/{pk}/']
>>> s.determine_path_prefix(paths)
'/api/v1'
>>> 

With the fix, this generates:

paths:
  /courses/:
    get:

[carltongibson]

Yes, super. Thank you for this @n2ygk