#### 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
getAjaxRequest().then(function (request) {
      var output = new Int8Array(request.params);
      expect(output.length).toEqual(2);
      expect(output[0]).toEqual(1);
      expect(output[1]).toEqual(2);
      done();
    });
  });
  it('should support binary data as array buffer view', function (done) {
    var input = new Int8Array(2);
    input[0] = 1;
    input[1] = 2;
    axios.post('/foo', input);
    getAjaxRequest().then(function (request) {
      var output = new Int8Array(request.params);
      expect(output.length).toEqual(2);
      expect(output[0]).toEqual(1);
      expect(output[1]).toEqual(2);
      done();
    });
  });
  it('should support array buffer response', function (done) {
    var response;
    function str2ab(str) {
      var buff = new ArrayBuffer(str.length * 2);
      var view = new Uint16Array(buff);
      for ( var i=0, l=str.length; i<l; i++) {
        view[i] = str.charCodeAt(i);
      }
      return buff;