 Support of http proxy

If I want to use http proxy，I will

```
 require('http').request({
    host:'127.0.0.1',
    port:8580,
    method:'POST',
    path:'https://www.google.com',
},function(res){
    //...
})
```

But I use node-fetch，

```
fetch(obj).then(function(res){
    ...
})
```

It throw `TypeError: Parameter 'url' must be a string, not object`

It does not support URL objects
