#### 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

test/specs/requests.spec.js
}
    axios('/foo', {
      responseType: 'arraybuffer'
    }).then(function (data) {
      response = data;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 200,
        response: str2ab('Hello world')
      });
      setTimeout(function () {
        expect(response.data.byteLength).toBe(22);
        done();
      }, 100);
    });
  });
  it('should support URLSearchParams', function (done) {
    var params = new URLSearchParams();
    params.append('param1', 'value1');
    params.append('param2', 'value2');
    axios.post('/foo', params);
    getAjaxRequest().then(function (request) {
      expect(request.requestHeaders['Content-Type']).toBe('application/x-www-form-urlencoded;charset=utf-8');
      expect(request.params).toBe('param1=value1&param2=value2');
      done();
    });
  });
});