#### 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/deepMerge.spec.js
var deepMerge = require('../../../lib/utils').deepMerge;
describe('utils::deepMerge', function () {
  it('should be immutable', function () {
    var a = {};
    var b = {foo: 123};
    var c = {bar: 456};
    deepMerge(a, b, c);
    expect(typeof a.foo).toEqual('undefined');
    expect(typeof a.bar).toEqual('undefined');
    expect(typeof b.bar).toEqual('undefined');
    expect(typeof c.foo).toEqual('undefined');
  });
  it('should deepMerge properties', function () {
    var a = {foo: 123};
    var b = {bar: 456};
    var c = {foo: 789};
    var d = deepMerge(a, b, c);
    expect(d.foo).toEqual(789);
    expect(d.bar).toEqual(456);
  });
  it('should deepMerge recursively', function () {
    var a = {foo: {bar: 123}};
    var b = {foo: {baz: 456}, bar: {qux: null}};
    expect(deepMerge(a, b)).toEqual({