Commented Unassigned: Invalid UsedRange of worksheet [9407]

Hi, I am not sure if it is a desired effect, but I get strange result using RangeUsed method of IXLWorksheet. My simple Excel file contains only two columns but with some special structure (look at the attachment).

I get __B1__:__B3__, but if I'm looking at cells included in this range (CellsUsed()) there are no cells from __A__ column. But... using the same method on worksheet everything seems to be OK. It looks like the range is not computed correctly (or at least its cells).

I retrieve cells using:
```
var rangeUsedAddresses = rangeUsed.CellsUsed()
.Select(c => c.Address)
.GroupBy(k => k.ColumnNumber).ToDictionary(k => k.Key, v => v.ToList());

// This one is correct
var worksheetAddresses = currentWorksheet.CellsUsed().
.Select(c => c.Address)
.GroupBy(k => k.ColumnNumber).ToDictionary(k => k.Key, v => v.ToList());
```
Comments: It's a bug. I can't believe I've never heard of it...