Fix test that was spuriously passing with sqlite3

sqlite3 does case-sensitive sorting, which means that "email" was sorting before "A name is now...", and therefore when no letter was specified, the users index was defaulting to 'E'.  With the switch to mysql, this broke the test because it was now defaulting to 'A'.

See https://github.com/edendevelopment/paginate_alphabetically/blob/master/lib/paginate_alphabetically/active_record.rb#L16-L20 for the code that does this.