fix a bug. print '-' when a cell is null to make the number of cells the
smae in each rows.
set cookie string on the reponse object.
1. supporting Excel 2007 format to download a excel file that has rows
more than 65,000.

2. reducing memory pressure by using SXSSF. SXSSF does not use DOM model
and write out rows when the number of rows are above a fixed window
size.
delete fileName parameter in ContentType.
delete ChannelBuffer
delete unused imports