Retrieve file after store

[david-zw-liu]

Brief Summary of Changes

Try to fix the problem, that model.image.file will be nil even if the file is uploaded. Calling retrieve_from_store! after store! called can populate @file and @identifier variable. So that will fix this problem.

What does this PR address?

• [x] GitHub issue (https://github.com/cloudinary/cloudinary_gem/issues/303)
• [ ] Refactoring
• [ ] New feature
• [ ] Bug fix
• [ ] Adds more tests

Are tests included?

• [x] Yes
• [ ] No

Reviewer, please note:

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [x] I ran the full test suite before pushing the changes and all the tests pass.

[const-cloudinary]

@david-liu-950627 , thank you for contribution!