xlrd.open_workbook accepts a number of parameters: http://www.lexicon.net/sjmachin/xlrd.html#xlrd.open_workbook-function

That being said I saw the kwd argument in the ExcelFile constrcutor. Currently that kwd is not used which means if the user tries to pass any additional parameters, or misspells them, they are silently dropped.

This change not only allows passing more arguments down to xlrd but also adds an additional runtime check.