Fixed #12982 -- Added a ``get_or_set`` method to the BaseCache backend.

https://code.djangoproject.com/ticket/12982
Actually, this note may be redundant since we use `cache.add` instead of `cache.set`. Here is a test script(not a great one I admit): https://gist.github.com/berkerpeksag/4f63000c32f86f56920b Is there something I'm missing?