dns: Use object without protoype for map

Currently we use `{}` for the `lookup` function to find the relevant
resolver to the dns.resolve function. It is preferable to use an
object without a Object.prototype, currently for example you can do
something like:

```js
dns.resolve("google.com", "toString", console.log);
```

And get `[Object undefined]` logged and the callback would never be
called. This is unexpected and strange behavior in my opinion.
In addition, if someone adds a property to `Object.prototype` might
also create unexpected results.

This pull request fixes it, with it an appropriate error is thrown.

PR-URL: https://github.com/nodejs/node/pull/5843


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michaël Zasso <mic.besace@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>