fix: remove stale Content-Length when body is set to ReadableStream or Response

[fengmk2]

Problem

When ctx.body is replaced with a ReadableStream or Response after being set to a string/buffer, the previously-set Content-Length header is not removed. This causes a Content-Length mismatch between the header and the actual body, which can lead to:

• Body truncation or padding
• Keep-alive connection corruption
• HTTP desync issues in proxy deployments

The existing Stream handler already correctly removes Content-Length when replacing a body, but the ReadableStream and Response handlers were missing this logic.

Fix

• ReadableStream handler: Add this.remove('Content-Length') when original != null (consistent with Stream handler)
• Response handler: Add this.remove('Content-Length') when original != null before copying headers from the Response object. This ensures any stale Content-Length from a previous body is cleared; if the Response itself has a Content-Length, it will be set correctly when its headers are copied.

Tests

Added two test cases in __tests__/response/body.test.js:

• Verify stale Content-Length is removed when body changes from string to ReadableStream
• Verify stale Content-Length is removed when body changes from string to Response

All 431 existing tests pass.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.90%. Comparing base (d3ea8bf) to head (01eb671).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1939   +/-   ##
=======================================
  Coverage   99.90%   99.90%           
=======================================
  Files           9        9           
  Lines        2094     2096    +2     
=======================================
+ Hits         2092     2094    +2     
  Misses          2        2           

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[Copilot]

Pull request overview

This PR fixes an HTTP response header correctness issue where Content-Length could become stale when replacing an already-sized body (e.g., a string) with a ReadableStream or a Response, and adds regression tests to prevent reintroduction.

Changes:

• Remove Content-Length when switching res.body to a ReadableStream (when there was a previous body).
• Remove Content-Length when switching res.body to a Response (when there was a previous body).
• Add tests asserting Content-Length is cleared in both replacement scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
lib/response.js	Clears stale Content-Length when body is replaced with ReadableStream or Response.
__tests__/response/body.test.js	Adds regression tests ensuring Content-Length is removed on those body replacements.

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[guoyangzhen]

Good catch. The Blob handler already sets this.length = val.size which implicitly overwrites Content-Length, but ReadableStream and Response handlers were silently keeping the stale value.

One thing to consider: the Blob handler at line 209 doesn't have this issue because it always sets this.length. But if you wanted to be extra consistent, you could also add if (original != null) this.remove('Content-Length') before this.length = val.size in the Blob handler — though functionally it's equivalent since the setter overwrites it anyway.

LGTM!