#### 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
response = res;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 200,
        statusText: 'OK',
        responseText: '{"foo": "bar"}',
        headers: {
          'Content-Type': 'application/json'
        }
      });
      setTimeout(function () {
        expect(response.data.foo).toEqual('bar');
        expect(response.status).toEqual(200);
        expect(response.statusText).toEqual('OK');
        expect(response.headers['content-type']).toEqual('application/json');
        done();
      }, 100);
    });
  });
  it('should not modify the config url with relative baseURL', function (done) {
    var config;
    axios.get('/foo', {
        baseURL: '/api'
    }).catch(function (error) {
        config = error.config;
    });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 404,