fix: ensure HTTP/2 sends Content-Length for empty POST requests

[mcollina]

This relates to...

Fixes #4594
Related to #4612

Rationale

This PR addresses the discrepancy in behavior between HTTP/1 and HTTP/2 when handling empty POST requests.

According to RFC 9110:

A user agent SHOULD send Content-Length in a request when the method defines a meaning for enclosed content and it is not sending Transfer-Encoding. For example, a user agent normally sends Content-Length in a POST request even when the value is 0 (indicating empty content).

Previously:

• HTTP/1: Empty POST requests correctly included Content-Length: 0
• HTTP/2: Empty POST requests omitted the Content-Length header entirely

This inconsistency causes compatibility issues with servers like Internet Information Server (IIS), which reject empty POST requests without a Content-Length header, returning 411 Length Required.

Changes

Implementation (lib/dispatcher/client-h2.js)

• Modified the logic to only omit Content-Length when both contentLength === 0 AND the method doesn't expect a payload (like GET, HEAD)
• For methods that expect payloads (POST, PUT, PATCH, etc.), Content-Length: 0 is now sent even when the body is empty
• Updated the assertion to allow contentLength === 0 without a body

Tests (test/fetch/http2.js)

• Updated the test to verify that empty POST requests with HTTP/2 now include Content-Length: 0
• Updated test name and comments to reflect the corrected behavior
• Both HTTP/1 and HTTP/2 tests now expect consistent behavior

Test Results

All HTTP/2 tests pass (51 tests total):

• ✅ [Fetch] Empty POST without h2 has Content-Length
• ✅ [Fetch] Empty POST with h2 has Content-Length

Breaking Changes and Deprecations

None. This is a bug fix that aligns behavior with RFC 9110 recommendations and HTTP/1 implementation.

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• Documented
• Review ready

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (96fb72c) to head (e83e4e3).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4613      +/-   ##
==========================================
- Coverage   92.93%   92.93%   -0.01%     
==========================================
  Files         106      106              
  Lines       32973    33091     +118     
==========================================
+ Hits        30645    30753     +108     
- Misses       2328     2338      +10     

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[metcoder95]

CI failures are unrelated

[mcollina]

@metcoder95 can I get a fresh approval?

[gurgunday]

lgtm