#### Summary
Trying to use Axios on Node with your own http(s)Agent set via the global default triggers the error: `TypeError: self.agent.addRequest is not a function`

For minimal example see: https://runkit.com/rupesh/59b1bcc1efc2f800128b7d54

It seems to be related to utils.merge not considering/handling the prototype chain:
<img width="814" alt="developer_tools_-_node_js" src="https://user-images.githubusercontent.com/3313870/30188235-58af3bd8-9425-11e7-9f99-4bc01ff4bf96.png">

Stack trace:
```
TypeError: self.agent.addRequest is not a function
at new ClientRequest in core _http_client.js — line 160
at http.Agent in core http.js — line 31
at RedirectableRequest._performRequest in follow-redirects/index.js — line 73
at Writable.RedirectableRequest in follow-redirects/index.js — line 57
at wrappedProtocol.request in follow-redirects/index.js — line 229
at dispatchHttpRequest in axios/lib/adapters/http.js — line 131
at httpAdapter in axios/lib/adapters/http.js — line 18
```

#### Context

- axios version: 0.16.2
- Environment: node v4, 6, 7 and 8

lib/core/Axios.js
'use strict';
var utils = require('./../utils');
var buildURL = require('../helpers/buildURL');
var InterceptorManager = require('./InterceptorManager');
var dispatchRequest = require('./dispatchRequest');
var mergeConfig = require('./mergeConfig');
/**
 * Create a new instance of Axios
 *
 * @param {Object} instanceConfig The default config for the instance
 */
function Axios(instanceConfig) {
  this.defaults = instanceConfig;
  this.interceptors = {
    request: new InterceptorManager(),
    response: new InterceptorManager()
  };
}
/**
 * Dispatch a request