update mongous.prototype.find function to let the mongous to be used widely

`i instanceof Object` is not a good way to check type.
when i use mongous on [node-webkit](https://github.com/rogerwang/node-webkit), the source code is running in `window`(web global context) , the `Object` is `window.Object` not `global.Object`(nodejs context), so check type is failed. 
so mongous can not be use in [node-webkit](https://github.com/rogerwang/node-webkit).
I fixed it.