ReadableWorkbook isDate1904 resolution problem

[vespesa]

The attached Excel file is very simple:

The file is saved in date1904 format, but ReadableWorkbook does not recognize that. On the other hand, Apache POI XSSFWorkbook resolves the date mode correctly (in Clojure REPL):

> (def f (clojure.java.io/file "window1904.xlsx"))
> (.isDate1904 (org.dhatim.fastexcel.reader.ReadableWorkbook. f))
false
> (.isDate1904 (org.apache.poi.xssf.usermodel.XSSFWorkbook. f))
true

window1904.xlsx

[rzymek]

fixed by #527