Fix finding worksheets with invalid chars in name

[FrediKats]

I was trying to write my own GetOrAdd method in my project and got test fails because of "[" and "]" symbols. I think that lib should have same behavior for "Add(name)" and "[name]". So i just add validator from "Add" method.

If we do not want to change indexer, i can add method GetOrAdd. For now, i cannot do this with public API (ValidateFixSheetName is internal).