fix(config): enable --overlay

[hiroppy]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

yes

Motivation / Use-Case

Currently, --overlay is not working at master branch. Please try to run examples/cli/overlay.

Breaking Changes

no

Additional Info

[alexander-akait]

Is it regression?

[hiroppy]

Yes. We were not aware of this bug. FYI: https://github.com/webpack/webpack-dev-server/tree/master/examples/cli/overlay

[alexander-akait]

Feel free to merge after CI green

[codecov]

Codecov Report

Merging #1968 into master will decrease coverage by 0.07%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1968      +/-   ##
==========================================
- Coverage   92.55%   92.47%   -0.08%     
==========================================
  Files          28       28              
  Lines        1075     1077       +2     
  Branches      321      322       +1     
==========================================
+ Hits          995      996       +1     
- Misses         76       77       +1     
  Partials        4        4

Impacted Files	Coverage Δ
lib/utils/createConfig.js	91.81% <50%> (-0.78%)	⬇️

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[codecov]

Codecov Report

Merging #1968 into master will decrease coverage by 0.07%.
The diff coverage is 50%.

@@            Coverage Diff             @@
##           master    #1968      +/-   ##
==========================================
- Coverage   92.55%   92.47%   -0.08%     
==========================================
  Files          28       28              
  Lines        1075     1077       +2     
  Branches      321      322       +1     
==========================================
+ Hits          995      996       +1     
- Misses         76       77       +1     
  Partials        4        4

Impacted Files	Coverage Δ
lib/utils/createConfig.js	91.81% <50%> (-0.78%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 0783a4a...0b5d972. Read the comment docs.