#### 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
var utils = require('../../../lib/utils');
var Stream = require('stream');
describe('utils::isX', function () {
  it('should validate Array', function () {
    expect(utils.isArray([])).toEqual(true);
    expect(utils.isArray({length: 5})).toEqual(false);
  });
  it('should validate Buffer', function () {
    expect(utils.isBuffer(Buffer.from('a'))).toEqual(true);
    expect(utils.isBuffer(null)).toEqual(false);
    expect(utils.isBuffer(undefined)).toEqual(false);
  });
  it('should validate ArrayBuffer', function () {
    expect(utils.isArrayBuffer(new ArrayBuffer(2))).toEqual(true);
    expect(utils.isArrayBuffer({})).toEqual(false);
  });
  it('should validate ArrayBufferView', function () {
    expect(utils.isArrayBufferView(new DataView(new ArrayBuffer(2)))).toEqual(true);
  });
  it('should validate FormData', function () {
    expect(utils.isFormData(new FormData())).toEqual(true);
  });
  it('should validate Blob', function () {
    expect(utils.isBlob(new Blob())).toEqual(true);
  });
  it('should validate String', function () {
    expect(utils.isString('')).toEqual(true);
    expect(utils.isString({toString: function () { return ''; }})).toEqual(false);
  });