#### 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/deepMerge.spec.js
foo: {
        bar: 123,
        baz: 456
      },
      bar: {
        qux: null
      }
    });
  });
  it('should remove all references from nested objects', function () {
    var a = {foo: {bar: 123}};
    var b = {};
    var d = deepMerge(a, b);
    expect(d).toEqual({
      foo: {
        bar: 123
      }
    });
    expect(d.foo).not.toBe(a.foo);
  });
  it('handles null and undefined arguments', function () {
    expect(deepMerge(undefined, undefined)).toEqual(undefined);
    expect(deepMerge(undefined, {foo: 123})).toEqual({foo: 123});
    expect(deepMerge({foo: 123}, undefined)).toEqual(undefined);
    expect(deepMerge(null, null)).toEqual(null);
    expect(deepMerge(null, {foo: 123})).toEqual({foo: 123});
    expect(deepMerge({foo: 123}, null)).toEqual(null);
  });
});