#### 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/utils.js
for (var i = 0, l = obj.length; i < l; i++) {
      fn.call(null, obj[i], i, obj);
    }
  } else {
    
    for (var key in obj) {
      if (Object.prototype.hasOwnProperty.call(obj, key)) {
        fn.call(null, obj[key], key, obj);
      }
    }
  }
}
/**
 * Accepts varargs expecting each argument to be an object, then
 * immutably merges the properties of each object and returns result.
 *
 * When multiple objects contain the same key the later object in
 * the arguments list will take precedence.
 *
 * Example:
 *
 * ```js
 * var result = merge({foo: 123}, {foo: 456});
 * console.log(result.foo); // outputs 456
 * ```
 *
 * @param {Object} obj1 Object to merge
 * @returns {Object} Result of all merge properties
 */
function merge() {