Fix non-existed file is improperly considered as existed when running collectstatic

[jxltom]

Step to reproduce:

• For example, if file_example2 already exists in OSS, exists() function will return True for file file_example even though it is not. Then collectstatic will try to get meta data of this file and then error will be raised since file does not actually exist.

Reason:

• It is because in Linux (maybe also in Windows), os.path.normpath will remove / which is at the end of path. For example, /path/path/ will be normalized to /path/path. So the exits() method which checks existence of a folder by self.bucket.list_objects(prefix='/path/path/', delimiter='', marker='', max_keys=1) is actually checking by self.bucket.list_objects(prefix='/path/path', delimiter='', marker='', max_keys=1) without / at the end of prefix

How to fix:

• It is fixed by appending / to the end of path if original path is ends with /.

[CLAassistant]

All committers have signed the CLA.

[huiguangjun]

merged

[jxltom]

Why this PR is merged without my author information. It is quite uncommon in open source.

[favoyang]

@jxltom, because the maintainer merged a combined PR (https://github.com/aliyun/django-oss-storage/pull/24), which makes you the indirect author, and not shown on https://github.com/aliyun/django-oss-storage/graphs/contributors.

Anyway, the project doesn't have a contributor file.

[jxltom]

Obviously, the merged PR could of course cherrypick my commits which includes my author information. It is much better way for open source projects.