allow different types of res_iter

[rafis]

Currently WSAPI accepts only function iterator as res_iter. This forces WSAPI frameworks and applications writers to return function iterator from their run(). Following code change allows users to use four different types of iterator: function, thread, table, string.

There is also small change for strictier check of not res to type(res) ~= "string", so for example if iterator returns some boolean true or table, it will no go to lfcgi.write() raising error at luaL_checklstring(), because it is not WSAPI error, it is application's error, application should return only strings from res_iter!

[hishamhm]

Looks like this is fully backwards compatible, right? Could you update the code so it merges cleanly?