Add tests for empty POST and content-length

[fredericDelaporte]

This relates to...

A discrepancy in behavior about empty POST request, when enabling or not enabling H2.
See #4594.

Rationale

This adds two tests:

• [Fetch] Empty POST without h2 has Content-Length
• [Fetch] Empty POST with h2 has no Content-Length

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).

So, the behavior of Undici sending a 0 Content-Length on empty POST request when H2 is not enabled is recommended by the RFC, while its behavior of sending no Content-Length on such requests when H2 is enabled does not match the recommendation. (It is still compliant since that is a "SHOULD", not a "MUST".)

Whether this should be changed or not, I do not know. Is there a good reason for this behavior with H2?

For your information, some servers cannot receive empty POST requests without a Content-Length. That is the case of Internet Information Server (IIS), the default web server of Windows OSes. Windows OSes underlying http.sys rejects such requests with a 411 Length required response.

Changes

Add two tests to demonstrate the current behavior of Undici.

Features

N/A

Bug Fixes

N/A

Breaking Changes and Deprecations

None

Status

• I have read and agreed to the Developer's Certificate of Origin
• Tested
• [S] Benchmarked (optional)
• Documented
• Review ready
• In review
• Merge ready

[fredericDelaporte]

I have not checked other verbs which semantics imply a content, like PUT, PATCH. If changing this behavior about POST, they should likely be checked too.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.93%. Comparing base (96fb72c) to head (ff97f0c).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4612      +/-   ##
==========================================
- Coverage   92.93%   92.93%   -0.01%     
==========================================
  Files         106      106              
  Lines       32973    32973              
==========================================
- Hits        30645    30644       -1     
- Misses       2328     2329       +1     

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