[Bugfix] fix sending response when header is sent

[Howard86]

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Bug fix

• What is the current behavior? (You can also link to an open issue here)

it will throw error if header is sent, e.g. we use res.direct in the handler

• What is the new behavior (if this is a feature change)?

it shall not throw error as we prevent calling res.send

• Other information:

[vercel[bot]]

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🔍 Inspect: https://vercel.com/howard86/next-api-handler/HF5j9hHXMn7iYevgGLmJYcUsG2X1
✅ Preview: https://next-api-handler-git-bugfix-fix-sending-respons-aae240-howard86.vercel.app

[cypress[bot]]

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Test summary

11 • 0 • 0 • 0

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Run details

Project	next-api-handler
Status	Passed
Commit	c35863a86d
Started	Feb 24, 2022 11:34 AM
Ended	Feb 24, 2022 11:34 AM
Duration	00:19 💡
OS	Linux Debian - 10.8
Browser	Electron 94

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[codecov[bot]]

Codecov Report

Merging #22 (c35863a) into main (fa5b3fe) will not change coverage. The diff coverage is 100.00%.

@@            Coverage Diff            @@
##              main       #22   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            3         3           
  Lines           98        99    +1     
  Branches        11        12    +1     
=========================================
+ Hits            98        99    +1     

Impacted Files	Coverage Δ
src/lib/router-builder.ts	100.00% <100.00%> (ø)

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[codeclimate[bot]]

Code Climate has analyzed commit c35863a8 and detected 0 issues on this pull request.

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