Add failing test which tries to save the same content twice

[melwas]

See: #367

@jschneier This is the failing tests.

With a new implementation Django saves intermediate files, so you can have two files with the same content but with different names (here is the closed issue on Django Issue Tracker).

You can see it here — self._save is called twice with the same content but with different names (it is not visible at first because hashed_name is recalculated just before save).

Django-Storages' code assumes that the content wasn't read before, but it is not the case with new Django's implementation. File content is already read, so the second read returns an empty bytes array.

I believe that django-storages should not make any assumptions regarding whether the file has been read before or not, and always seek(0) before reading.

What do you think?