#### 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/headers.spec.js
axios('/foo');
    getAjaxRequest().then(function (request) {
      for (var key in headers) {
        if (headers.hasOwnProperty(key)) {
          expect(request.requestHeaders[key]).toEqual(headers[key]);
        }
      }
      done();
    });
  });
  it('should not set header if value is null', function (done) {
    expect(axios.defaults.headers.common['Accept']).toEqual('application/json, text/plain, */*');
    axios('/foo', {
      headers: {
        Accept: null
      }
    });
    getAjaxRequest().then(function (request) {
      expect(typeof request.requestHeaders['Accept']).toEqual('undefined');
      done();
    });
  });
  it('should add extra headers for post', function (done) {
    var headers = axios.defaults.headers.common;
    axios.post('/foo', 'fizz=buzz');
    getAjaxRequest().then(function (request) {
      for (var key in headers) {
        if (headers.hasOwnProperty(key)) {
          expect(request.requestHeaders[key]).toEqual(headers[key]);
        }