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. react/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.