Don't rely on process.env.hasOwnProperty

Don't rely on process.env.hasOwnProperty as in Node <0.9.0 versions it doesn't work like a proper JS object.

```
$ node -v
v0.8.28
$ node
> typeof process.env
'object'
> typeof process.env.hasOwnProperty
'undefined'
```

See: https://github.com/joyent/node/issues/3521