UnicodeError while post processing

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In django-staticfiles/staticfiles/storage.py, post_processing, the original_file is just read but not decoded, leading to UnicodeError if the original_file contains non-ascii characters.

https://github.com/jezdez/django-staticfiles/blob/develop/staticfiles/storage.py#L266

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However, it may be not possible to determine the correct encoding of the file. Maybe making the default to 'utf-8' is better.