URLPathVersioning: Use Default Version when It's None (Not Invalid)

[ykh]

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

Allowed Versions: v1, v2
Default Version: v2 (It's latest version as often)
My URL Pattern: api/((?P<version>(\w){2})/)?users/

Expected behavior

1- When GET api/v1/users/: get users data according to version v1.
2- When GET api/users/ : get users data according to Default-Version.

Actual behavior

for Case 1: get users data according to version v1.
for Case 2: get 404 error: "Invalid version in URL path."

I think it makes sense when client didn't specified version in URI, he expects get response from latest version as often (not getting error)!

[lovelydinosaur]

Sanity check: Does the second version populate the URL keyword arguments as {"version": None}, or {}?

[ykh]

@tomchristie "version" is None.