Update ExportGrailsPlugin.groovy

I need to implement a specific my Excel exporter. I found in the file ExportGrailsPlugin.grovvy, an undocumented possibility of setting up your own Excel exporter. Unfortunately there is a bug that sends the application to crash if I set in config.groovy the custome exporter. The fix is ​​very simple and consists in modifying the grailsApplication attribute to application in line 44. The problem is that i'm using grails 1.3.7 and i can upgrade to version 1.1 at the maximum.
Merge pull request #1 from cristallo/patch-1

Update ExportGrailsPlugin.groovy