SetCellValue: use fmt.Sprint(v) instead of fmt.Sprintf("%v", v)

Because that does the same thing, but without having to parse a format
string.

Signed-off-by: Olivier Mengué <dolmen@cpan.org>

cell.go

@@ -94,7 +94,7 @@ func (f *File) SetCellValue(sheet, axis string, value interface{}) error {
 	case nil:
 		err = f.SetCellStr(sheet, axis, "")
 	default:
-		err = f.SetCellStr(sheet, axis, fmt.Sprintf("%v", value))
+		err = f.SetCellStr(sheet, axis, fmt.Sprint(value))
 	}
 	return err
 }