#### 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

lib/core/dispatchRequest.js
/**
 * Dispatch a request to the server using the configured adapter.
 *
 * @param {object} config The config that is to be used for the request
 * @returns {Promise} The Promise to be fulfilled
 */
module.exports = function dispatchRequest(config) {
  throwIfCancellationRequested(config);
  
  config.headers = config.headers || {};
  
  config.data = transformData(
    config.data,
    config.headers,
    config.transformRequest
  );
  
  config.headers = utils.merge(
    config.headers.common || {},
    config.headers[config.method] || {},
    config.headers
  );
  utils.forEach(
    ['delete', 'get', 'head', 'post', 'put', 'patch', 'common'],
    function cleanHeaderConfig(method) {
      delete config.headers[method];
    }
  );
  
  utils.forEach(config.headers, function deleteNullValueHeaders(value, key) {