Created Unassigned: Regression in spreadsheet import [4614]

Regression in version 1.0.0.57 for importing spreadsheets. Reverting to 1.0.0.56 resolves the issue.

Invalid Cast Exception - Unable to cast object of type System.Collections.Generic.IEnumerable<System.String> to System.Collections.Generic.IList<System.Collections.Generic.IList<System.String>> at tomenglertde.ResXManager.Model.ResourceEntityExtensions.ImportMultipleSheets

Issue appears to be at line 203 of ExcelExtensions-

```
var data = (IList<IList<string>>)rows.Select(row => row.GetCellValues(sharedStrings)).ToArray();
```

Interestingly neither ImportMultipleSheets or the GetCellValues method seem to have changed between 1.0.0.56 and 1.0.0.57