Support HTTP/2

[cainrus]

This PR contains a:

• bugfix
• new feature
• code refactor
• test update
• typo fix
• metadata update

Motivation / Use-Case

webpack-hot-middleware must to support HTTP/2 out of the box.

Breaking Changes

Nope.

Additional Info

It's impossible to cover with tests HTTP/2 mode right now because Express HTTP/2 support is in progress: expressjs/express#3390

[jsf-clabot]

All committers have signed the CLA.

[codecov]

Codecov Report

Merging #322 into master will increase coverage by 0.21%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #322      +/-   ##
==========================================
+ Coverage    87.5%   87.71%   +0.21%     
==========================================
  Files           3        3              
  Lines         232      236       +4     
  Branches       65       66       +1     
==========================================
+ Hits          203      207       +4     
  Misses         29       29

Impacted Files	Coverage Δ
middleware.js	92.95% <100%> (+0.42%)	⬆️

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