Object Reference not Set (Cell.cs) about excellibrary HOT 2 CLOSED

New test case is created at r21.

I think we don't need the set accessor for Cell.DateTimeValue. The DateTime 
value can
be set directly when calling the Cell constructor with current codebase. When 
saving
the workbook, all cells are encoded again using WorkSheetEncoder.EncodeCell() 
anyway.

However, calling other properties such Cell.Format and Cell.BackColor also cause
NullReferenceException be thrown for newly created worksheeet.

Fixed by r24.