fixed critical bug: `process.env` flattens all env-vars.

Cluster clients can not receive deep-level objects or arrays in the forked process, e.g.:
```javascript
{
  "args": [
    "foo",
    "bar"
  ], 
  "env": { 
    "foo1": "bar1" 
  }
} 
```

will be parsed to
```javascript
{
  "args": "foo, bar", 
  "env": "[object Object]"
}
```

Actually it's a `process.env` problem.

#896