.eql() is broken in ES6

Following tests should not pass, but pass in Node v4+ and [modern browsers that support ES6](https://kangax.github.io/compat-table/es6/#test-Object_static_methods_accept_primitives_Object.keys).

``` js
expect('a').to.be.eql(['a']);
expect('a').to.be.eql({0: 'a'});
expect(1).to.be.eql({});
expect(true).to.be.eql({});
```

Change of Object.keys in ES6 causes this issue.
I've fixed same issues in [Node.js core assert.deepEqual](https://github.com/nodejs/node/pull/193) and [commonjs-assert](https://github.com/defunctzombie/commonjs-assert/pull/7).