VM: Unexpected behavior in vm.runInContext 

``` js
'use strict';
const vm = require('vm');

let obj = {};

const ctx = vm.createContext(obj);
vm.runInContext('name = 1', ctx);
console.log(obj);
vm.runInContext('function name() {}', ctx);
console.log(obj);

```

gives

```
{ name: 1 }
{ name: 1 }
```

`vm.runInContext` fails to overwrite `name` property of given context.

Tested versions: `v4.1.1` & `v4.2.1`
