[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