fix: do not call the next middleware for 304 response

[alexander-akait]

This PR contains a:

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

Motivation / Use-Case

fixes - webpack/webpack-dev-server#5519

Breaking Changes

Mostly no, potentially some application allow to change status code and remove headers, but webpack-dev-middleware should be the last point in middlewares, so this change fixes bugs when other middleware is trying to write headers/body

Additional Info

Inspired by - https://github.com/pillarjs/send/blob/master/index.js#L273

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.30%. Comparing base (0ac461b) to head (517a569).
⚠️ Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2155      +/-   ##
==========================================
+ Coverage   96.29%   96.30%   +0.01%     
==========================================
  Files          13       13              
  Lines         890      893       +3     
  Branches      265      266       +1     
==========================================
+ Hits          857      860       +3     
  Misses         33       33              

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