Adding forceArchive option.

Usefull option in download if you wanna keep the folders and architecture while you have only 1 real file.

( needed for Evaluation in order to download copies, => one folder with the student name is needed )
so without this patch actualy, if there is only 1 file in the directory, this download directly the file without the folder.

ex:
hidden directory for Evaluation1
---- copy_1_Paul_Pale
        ------ mydrop.txt

=> download result is 
only mydrop.txt


what is expected is the same file  structure that in claroline.