Exception when creating a spreadsheet with many date cells.

[bwilson]

java.lang.IllegalStateException: The maximum number of cell styles was exceeded. You can define up to 4000 styles in a .xls workbook.
at org.apache.poi.hssf.usermodel.HSSFWorkbook.createCellStyle(HSSFWorkbook.java:1158)
at org.apache.poi.hssf.usermodel.HSSFWorkbook.createCellStyle(HSSFWorkbook.java:76)
at org.grails.plugins.excelimport.ExcelImportService.setCellValue(ExcelImportService.groovy:591)

The ExcelImportService is creating a new CellStyle for each date cell, and thus exceeds the limit of 4,000 styles. I am about to check in a proposed fix.