[reader.excel] Regression test for files with None encoding

Created originally on Bitbucket by jamesbroadhead

Was already merged in Bitbucket before import, marked as merged by the import user

This adds an explicit regression test for https://bitbucket.org/openpyxl/openpyxl/issue/433 following the fix here https://bitbucket.org/openpyxl/openpyxl/commits/488b1ff05e4c9634f2b8230d5da6a31bc471b291

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