#### 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
c[key] = { user: 'foo', pass: 'test' }
      expect(mergeConfig(a, b)).toEqual(c);
    });
    it('should merge ' + key + ' with defaults', function() {
      var a = {}, b = {}, c = {};
      a[key] = { user: 'foo', pass: 'bar' };
      b[key] = { pass: 'test' };
      c[key] = { user: 'foo', pass: 'test' };
      expect(mergeConfig(a, b)).toEqual(c);
    });
    it('should overwrite default ' + key + ' with a non-object value', function() {
      [false, null, 123].forEach(function(value) {
        var a = {}, b = {}, c = {};
        a[key] = { user: 'foo', pass: 'test' };
        b[key] = value;
        c[key] = value;
        expect(mergeConfig(a, b)).toEqual(c);
      });
    });
  });
  it('should allow setting other options', function() {
    var merged = mergeConfig(defaults, { timeout: 123 });
    expect(merged.timeout).toEqual(123);
  });
  it('should allow setting custom options', function() {
    var merged = mergeConfig(defaults, { foo: 'bar' });
    expect(merged.foo).toEqual('bar');
  });
});