fix(alb): alb not working when header value is not string

[H4ad]

Fixes CodeGenieApp/serverless-express#563

Description of change

Force all headers to be string when it returns to the cloud.

Pull-Request Checklist

• Code is up-to-date with the main branch
• npm run lint passes with this change
• npm run test passes with this change
• This pull request links relevant issues as Fixes #0000
• There are new or updated unit tests validating the change
• Added documentation inside www/docs/main folder.
• Included new files inside index.doc.ts.
• The new commits follow conventions outlined in the conventional commit spec

[codecov]

Codecov Report

Merging #74 (10d72ac) into main (6edb996) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #74   +/-   ##
=======================================
  Coverage   99.68%   99.68%           
=======================================
  Files          60       60           
  Lines        1275     1275           
  Branches      292      292           
=======================================
  Hits         1271     1271           
  Misses          4        4           

Impacted Files	Coverage Δ
src/core/headers.ts	100.00% <100.00%> (ø)

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[github-actions]

🎉 This PR is included in version 2.14.1 🎉

The release is available on:

• npm package (@latest dist-tag)
• GitHub release

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