Custom cert chain option for underlying request

[robfromca]

Add an additional option so you can pass certificates through
to the underlying request, for connecting to hosts with non-standard
or custom certificate chains.

[gstroup]

Hey, how's it going? This change looks logical to me, but it's breaking some tests. Do you have time to fix tests, and maybe add one for your new case? I will try to take a look soon and merge this in. Thanks!

[gstroup]

Just ran the build again, and no failures. Looks like some other tests are flaky.

[monkpow]

@robfromca @gstroup I don't see this as a big advantage over

app.use('/', proxy('internalhost.example.com', {
  decorateRequest: function(proxyReq, originalReq) {
    proxyReq.ca =  [caCert, intermediaryCert]
    return proxyReq;
  }
})

and I don't see a specific way to test it -- we could verify that options.ca is populated, but I don't know how to write a test to verify that the certificate chain is functional.

For my money, I'd prefer to add documentation that this is a technique for adding certificate chains than take on additional code.

Happy to discuss more/be further educated ...

[robfromca]

I'll give your suggestion a shot, that's all I was trying to do and didn't think of trying that. If it works I'll make a PR for the docs.