#### 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
var rejectSpy = jasmine.createSpy('reject');
    axios('/foo', {
      validateStatus: function (status) {
        return status === 500;
      }
    }).then(resolveSpy)
      .catch(rejectSpy)
      .then(function () {
        expect(resolveSpy).toHaveBeenCalled();
        expect(rejectSpy).not.toHaveBeenCalled();
        done();
      });
    getAjaxRequest().then(function (request) {
      request.respondWith({
        status: 500
      });
    });
  });
  it('should resolve when the response status is 0 (i.e. requesting with file protocol)', function (done) {
    var resolveSpy = jasmine.createSpy('resolve');
    var rejectSpy = jasmine.createSpy('reject');
    axios('file:///xxx').then(resolveSpy)
      .catch(rejectSpy)
      .then(function () {
        expect(resolveSpy).toHaveBeenCalled();
        expect(rejectSpy).not.toHaveBeenCalled();
        done();
      });
    getAjaxRequest().then(function (request) {
      request.respondWith({