Client Side Certs with session

Summary.

Trying to get client side certificates to work with session but I'm seeing a difference between the results when making the call with a session and the helper get method.

Expected Result

Calling get with a Session returns the same result as calling requests.get.

Actual Result

Request fails as if the certificate was not passed.

Reproduction Steps

System Information

$ python -m requests.help

(panopta_env) ➜  ~ python -m requests.help
{
  "chardet": {
    "version": "3.0.4"
  },
  "cryptography": {
    "version": ""
  },
  "idna": {
    "version": "2.8"
  },
  "implementation": {
    "name": "CPython",
    "version": "2.7.16"
  },
  "platform": {
    "release": "18.6.0",
    "system": "Darwin"
  },
  "pyOpenSSL": {
    "openssl_version": "",
    "version": null
  },
  "requests": {
    "version": "2.22.0"
  },
  "system_ssl": {
    "version": "1000212f"
  },
  "urllib3": {
    "version": "1.25.3"
  },
  "using_pyopenssl": false
}

Start by reproducing the client-certificate request with a Session and with requests.get, using the issue's Python 2.7 and Requests 2.22.0 context. Compare how the certificate is supplied in both calls and add a regression test once the relevant request path is located; done means Session and requests.get produce the same authenticated result.