#### 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:

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
#### 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 functionFor minimal example see: https://runkit.com/rupesh/59b1bcc1efc2f800128b7d54
It seems to be related to utils.merge not considering/handling the prototype chain:

Stack trace:
#### Context