fixed bug in excel output format.
Multivalued fields would not be properly converted to the excel format.

Before this patch, only the last value in a multiple valued property would
be stored in the excel file. Now however, these properties are represented
as comma seperated entries, just like the CSV export functionality in SMW.

Change-Id: I7a45c93b303ba94b648d8d3171b14a2d063c9e1e
Merge "fixed bug in excel output format. Multivalued fields would not be properly converted to the excel format."