refact: eliminate duplication in Request/Response via struct embedding

[ksw2000]

Reduces code duplication by embedding the header struct into both RequestHeader and ResponseHeader to share common fields.

I found that there are many duplicate methods in header.go. We can reduce this repetition by using struct embedding. This PR improves maintainability by allowing shared logic between RequestHeader and ResponseHeader to be implemented in a single method with a single comment.

Additionally, by embedding the Header struct, it implicitly communicates to users which methods on RequestHeader and ResponseHeader share the exact same underlying implementation.

During the refactoring process, I also discovered that the SetProtocol methods in RequestHeader and ResponseHeader are inconsistent.

// SetProtocol sets HTTP request protocol.
func (h *RequestHeader) SetProtocol(method string) {
	h.protocol = append(h.protocol[:0], method...)
	h.noHTTP11 = !bytes.Equal(h.protocol, strHTTP11)
}

// SetProtocolBytes sets HTTP request protocol.
func (h *RequestHeader) SetProtocolBytes(method []byte) {
	h.protocol = append(h.protocol[:0], method...)
	h.noHTTP11 = !bytes.Equal(h.protocol, strHTTP11)
}

// SetProtocol sets response protocol bytes.
func (h *ResponseHeader) SetProtocol(protocol []byte) {
	h.protocol = append(h.protocol[:0], protocol...)
}

The same function name SetProtocol in RequestHeader and ResponseHeader have different inputs, and the ResponseHeader.SetProtocol() does not update the noHTTP11.

[Copilot]

Pull Request Overview

This PR refactors header handling by embedding the common header struct into both RequestHeader and ResponseHeader to eliminate code duplication and improve maintainability. Key changes include updating method calls to use the new ContentLength implementation and adjusting test initialization for secure logging behavior.

• Replace calls to realContentLength() with ContentLength() in Request.
• Update test initialization to assign secureErrorLogMessage via field assignment.

Reviewed Changes

Copilot reviewed 2 out of 3 changed files in this pull request and generated no comments.

File	Description
http.go	Updated method calls to use ContentLength() for header length checks
header_test.go	Changed struct initialization for secureErrorLogMessage in tests

Comments suppressed due to low confidence (2)

header_test.go:2784

• [nitpick] The change from initializing secureErrorLogMessage via composite literal to a separate field assignment is not immediately obvious; please add a brief inline comment explaining the necessity of this change.

	h := &ResponseHeader{}

http.go:1294

• Verify that the new ContentLength() covers all cases previously handled by realContentLength(). If there are any behavioral differences, consider documenting them.

	contentLength := req.Header.ContentLength()

[erikdubbelboer]

Thanks!