Fix genSortedIndex for tables with table/bool keys
Fix genSortedIndex for tables with table/bool keys

Previously it would assume the table contains only number and string
keys, and use table.sort's default comparison to sort the number and
string keys separate. Now the code uses a comparison function which
works for all types and also puts an order on the types.

Fixes bluebird75/luaunit#29

Reviewed-by: NiteHawk <5758ffa791ed0666fb75085c5624d5e6d66bbc05@users.noreply.github.com>

GuntherStruyf/luaunit,GuntherStruyf/luaunit