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.

lib/dns.js

@@ -243,7 +243,7 @@ function resolver(bindingName) {
 }
 
 
-var resolveMap = {};
+var resolveMap = Object.create(null);
 exports.resolve4 = resolveMap.A = resolver('queryA');
 exports.resolve6 = resolveMap.AAAA = resolver('queryAaaa');
 exports.resolveCname = resolveMap.CNAME = resolver('queryCname');