#### 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/core/mergeConfig.spec.js
expect(merged).not.toBe(defaults);
    expect(merged.headers).not.toBe(defaults.headers);
  });
  it('should allow setting request options', function() {
    var config = {
      url: '__sample url__',
      method: '__sample method__',
      params: '__sample params__',
      data: { foo: true }
    };
    var merged = mergeConfig(defaults, config);
    expect(merged.url).toEqual(config.url);
    expect(merged.method).toEqual(config.method);
    expect(merged.params).toEqual(config.params);
    expect(merged.data).toEqual(config.data);
  });
  it('should not inherit request options', function() {
    var localDefaults = {
      method: '__sample method__',
      data: { foo: true }
    };
    var merged = mergeConfig(localDefaults, {});
    expect(merged.method).toEqual(undefined);
    expect(merged.data).toEqual(undefined);
  });
  ['auth', 'headers', 'params', 'proxy'].forEach(function(key) {
    it('should set new config for' + key + ' without default', function() {
      var a = {}, b = {}, c = {}
      a[key] = undefined
      b[key] = { user: 'foo', pass: 'test' }