[BUG] (simulator) Behavior when multiple lines have the same header.

[bungoume]

Describe the problem
Fastly seems to exhibit a special behavior when there are multiple lines with the same header.
https://fiddle.fastly.dev/fiddle/6221d4db

While the specific behavior needs to be verified accurately, I'll write test cases based on the current understanding.

• Only the Header listed at the beginning is utilized:
  • When assigned, the value from the first instance is used.
  • Conditions such as 'if' statements use the value from the initial entry.
  • Writing using : such as set req.http.HEADER:V = ""; overwrites the first header entry, erasing subsequent entries from the second onwards.
• Header sequence reverses during the fetch process.
• Regardless of the presence of multiple lines, using unset removes all lines.
• Even with multiple lines, using set retains only that single line.
• At the stage of the Response from Fastly, multiple lines consolidate into a single header separated by commas.

VCL code that cause the problem / reproduceable
https://fiddle.fastly.dev/fiddle/aa00bc57

// @scope: recv
// @suite: ADD header(add-add-add) BUGGY
sub test_recv {
    add req.http.VALUE = "V1";
    add req.http.VALUE = "V2";
    add req.http.VALUE = "V3";
    # assert.equal(req.http.VALUE, "V1, V2, V3");  # request upstream with 3 line headers
    # assert.equal(resp.http.VALUE, "V1, V2, V3");  # response contains single line header with three values.

    set req.http.MESSAGE = req.http.VALUE;  # set first header value
    assert.equal(req.http.MESSAGE, "V1");
}

// @scope: recv
// @suite: ADD header(set-add-add) BUGGY
sub test_recv {
    set req.http.VALUE = "V1";
    add req.http.VALUE = "V2";
    add req.http.VALUE = "V3";
    # assert.equal(req.http.VALUE, "V1, V2, V3");  # request upstream with 3 headers

    set req.http.MESSAGE = req.http.VALUE;  # set first header value
    assert.equal(req.http.MESSAGE, "V1");
}

// @scope: recv
// @suite: ADD header(add-add-set)
sub test_recv {
    add req.http.VALUE = "V1";
    add req.http.VALUE = "V2";
    set req.http.VALUE = "V3";
    assert.equal(req.http.VALUE, "V3");  # 1 header

    set req.http.MESSAGE = req.http.VALUE;
    assert.equal(req.http.MESSAGE, "V3");
}

// @scope: recv
// @suite: UNSET header(add-add-unset)
sub test_recv {
    add req.http.VALUE = "V1";
    add req.http.VALUE = "V2";
    unset req.http.VALUE;
    assert.is_notset(req.http.VALUE);  # 0 header

    set req.http.MESSAGE = req.http.VALUE;
    assert.equal(req.http.MESSAGE, "(null)");
}

Additional context
https://github.com/bungoume/falco-vcl-empty-test/blob/main/tests/header_multiline.test.vcl