BytesIO TypeError

This fixes #3.

Thanks to @AndrewPashkin for the original test, and @zsiciarz for pointing me in the correct direction for fixing this.

https://github.com/AndrewPashkin/dj-inmemorystorage/commit/74964d9db0c6f7396d92b2942a41e61aef6587bb

https://github.com/zsiciarz/dj-inmemorystorage/commit/1c30c386ab6c64cb5cf4edc060c9f917ef174236