Fix HTTP method for downloading spreadsheet CSV
Fix HTTP method for downloading spreadsheet CSV

`button_to` defaults to use HTTP POST whereas the
`download_grader_students_mapping` action is a GET action.

Tested:
- local server

melneubert/Markus,david-yz-liu/Markus,reidka/Markus,wkwan/Markus,WilsonChiang/Markus,ealonas/Markus,reidka/Markus,jeremygoh/Markus,ealonas/Markus,reidka/Markus,SoftwareDev/Markus,ealonas/Markus,david-yz-liu/Markus,MarkUsProject/Markus,Lysette/Markus,jeremygoh/Markus,SoftwareDev/Markus,finnergizer/Markus,binuri/Markus,arkon/Markus,melneubert/Markus,wkwan/Markus,arkon/Markus,finnergizer/Markus,melneubert/Markus,SoftwareDev/Markus,david-yz-liu/Markus,reidka/Markus,SoftwareDev/Markus,MarkUsProject/Markus,wkwan/Markus,SoftwareDev/Markus,finnergizer/Markus,melneubert/Markus,finnergizer/Markus,WilsonChiang/Markus,finnergizer/Markus,MarkUsProject/Markus,benjaminvialle/Markus,MarkUsProject/Markus,finnergizer/Markus,reidka/Markus,melneubert/Markus,finnergizer/Markus,ealonas/Markus,arkon/Markus,MarkUsProject/Markus,ealonas/Markus,binuri/Markus,SoftwareDev/Markus,binuri/Markus,reidka/Markus,binuri/Markus,WilsonChiang/Markus,binuri/Markus,david-yz-liu/Markus,Lysette/Markus,ealonas/Markus,binuri/Markus,benjaminvialle/Markus,wkwan/Markus,jeremygoh/Markus,binuri/Markus,melneubert/Markus,ealonas/Markus,benjaminvialle/Markus,ealonas/Markus,david-yz-liu/Markus,david-yz-liu/Markus,jeremygoh/Markus,jeremygoh/Markus,binuri/Markus,melneubert/Markus,WilsonChiang/Markus,jeremygoh/Markus,binuri/Markus,SoftwareDev/Markus,WilsonChiang/Markus,david-yz-liu/Markus,finnergizer/Markus,jeremygoh/Markus,Lysette/Markus,arkon/Markus,benjaminvialle/Markus,Lysette/Markus,finnergizer/Markus,wkwan/Markus,ealonas/Markus,david-yz-liu/Markus,MarkUsProject/Markus,WilsonChiang/Markus,reidka/Markus,arkon/Markus,Lysette/Markus,WilsonChiang/Markus,MarkUsProject/Markus,reidka/Markus,arkon/Markus,melneubert/Markus,wkwan/Markus,melneubert/Markus,benjaminvialle/Markus,jeremygoh/Markus,Lysette/Markus,reidka/Markus,arkon/Markus,benjaminvialle/Markus,arkon/Markus,wkwan/Markus,Lysette/Markus,jeremygoh/Markus,benjaminvialle/Markus,WilsonChiang/Markus,MarkUsProject/Markus,WilsonChiang/Markus,david-yz-liu/Markus,arkon/Markus,wkwan/Markus,wkwan/Markus