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

[Tjatse]

Cluster clients can not receive deep-level objects or arrays in the forked process, e.g.:

{
  "args": [
    "foo",
    "bar"
  ], 
  "env": { 
    "foo1": "bar1" 
  }
} 

will be parsed to

{
  "args": "foo, bar", 
  "env": "[object Object]"
}

Actually it's a process.env problem.
#896

[rangercyh]

what about forkmode, it just work wrong as before
"env": {
"type": "conn",
"id": "conn-1",
"host": "10.20.127.197",
"clientPort": 5000,
"router": 3000
}

in the process,I print the process.env
env: '[object Object]',
how can I figure it out?