Handling infinity results in a graceful manner.

This fixes the bug that occurs when the result contains INF values.

When INF value is encountered, the exception occurs in line:

`cell.Value = Convert.ChangeType(cell.Value, crs.Columns[columnIndex].Type ?? ConvertXmlTypeToType(cell.Type), CultureInfo.InvariantCulture);`

since `cell.Value == "INF"`

The change treats invalid values as nulls (almost -- `cellsIndexer` still has to advance, since the value is provided in this case).