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

Merged Bitbucket Importer requested to merge bitbucket/merged-pr-57 into branch/2.2

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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