sso_proxy: add test for websockets and update docs

[Jusshersmith]

Problem

We've recently introduced support for websockets, however we didn't include any tests or documentation updates in the initial pull request.

Solution

Add in a couple of simple tests and documentation updates. Also, the docker golang image that CircleCI uses has been updated to 1.12 to bring it in line with SSO's go version (required for websocket support)

[codecov]

Codecov Report

Merging #256 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #256   +/-   ##
=======================================
  Coverage   62.04%   62.04%           
=======================================
  Files          50       50           
  Lines        4105     4105           
=======================================
  Hits         2547     2547           
  Misses       1370     1370           
  Partials      188      188

[benjsto]

Good test - this will be great to have if/when we do decide to move the hijack to a different handler.