utils.merge not considering/handling the prototype chain

[rupesh1]

#### 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

[rupesh1]

In hindsight this maybe another manifestation of: #812

[KamiKillertO]

🤔 it's seems that utils.merge erase the httpAgent prototype.

[david4City]

Anything on this subject ?

[KamiKillertO]

There's a pending PR (#1295) that should fix this issue.

[cybercussion]

I got this error on a self-signed cert. Initially I attempted to use httpsAgent: { rejectUnauthorized: false } but then opted to use httpsAgent: new https.Agent({ rejectUnauthorized: false }) and the error went away. Case someone else has that issue.