#### 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/utils/isX.spec.js
it('should validate Number', function () {
    expect(utils.isNumber(123)).toEqual(true);
    expect(utils.isNumber('123')).toEqual(false);
  });
  it('should validate Undefined', function () {
    expect(utils.isUndefined()).toEqual(true);
    expect(utils.isUndefined(null)).toEqual(false);
  });
  it('should validate Object', function () {
    expect(utils.isObject({})).toEqual(true);
    expect(utils.isObject(null)).toEqual(false);
  });
  it('should validate Date', function () {
    expect(utils.isDate(new Date())).toEqual(true);
    expect(utils.isDate(Date.now())).toEqual(false);
  });
  it('should validate Function', function () {
    expect(utils.isFunction(function () {})).toEqual(true);
    expect(utils.isFunction('function')).toEqual(false);
  });
  it('should validate Stream', function () {
    expect(utils.isStream(new Stream.Readable())).toEqual(true);
    expect(utils.isStream({ foo: 'bar' })).toEqual(false);
  });
  it('should validate URLSearchParams', function () {
    expect(utils.isURLSearchParams(new URLSearchParams())).toEqual(true);
    expect(utils.isURLSearchParams('foo=1&bar=2')).toEqual(false);
  });
});