Handle external upgrade for all websocket proxies

[jamesblight]

This change upgrades websocket proxies so that an inital http request doesn't have to be made.
https://github.com/chimurai/http-proxy-middleware#external-websocket-upgrade

It supports a proposed change in Create React App by handling the upgrade of any websocket proxy passed in. facebook/create-react-app#1790

I added a test to confirm the upgrade

[jsf-clabot]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #843 into master will increase coverage by 0.33%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #843      +/-   ##
==========================================
+ Coverage   72.72%   73.05%   +0.33%     
==========================================
  Files           4        4              
  Lines         407      412       +5     
  Branches      119      120       +1     
==========================================
+ Hits          296      301       +5     
  Misses        111      111

Impacted Files	Coverage Δ
lib/Server.js	82.97% <100%> (+0.31%)	✅

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[SpaceK33z]

Awesome!! Thanks, I'll release a new version today.

[SpaceK33z]

This is published in version 2.4.2.

[shellscape]

@jamesblight in working on version 3, we've removed SockJS for sockets across the board, instead replacing it with ws for a WebSocket Server, and native WebSocket on the client. Unfortunately, that move has "broken" tests for proxying WebSockets. The test has been moved here, on the refactor-lib branch https://github.com/webpack/webpack-dev-server/blob/refactor-lib/test/tests/proxy.js#L199. Any chance you would be willing to take a look a this setup? I've been so far unsuccessful in getting it to work.