#### 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
function testHeaderValue(headers, key, val) {
  var found = false;
  for (var k in headers) {
    if (k.toLowerCase() === key.toLowerCase()) {
      found = true;
      expect(headers[k]).toEqual(val);
      break;
    }
  }
  if (!found) {
    if (typeof val === 'undefined') {
      expect(headers.hasOwnProperty(key)).toEqual(false);
    } else {
      throw new Error(key + ' was not found in headers');
    }
  }
}
describe('headers', function () {
  beforeEach(function () {
    jasmine.Ajax.install();
  });
  afterEach(function () {
    jasmine.Ajax.uninstall();
  });
  it('should default common headers', function (done) {
    var headers = axios.defaults.headers.common;