Revert "Content-Length header not always returned when enumerating HttpContentHeaders"

[ManickaP]

Reverts #102416

The change broke outerloop tests in System.Net.Tests.SyncWebClientTest. Seems like it sends bogus Content-Length:

cc @pedrobsaila @MihaZupan

[ManickaP]

/azp run runtime-libraries-coreclr outerloop

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[MihaZupan]

@pedrobsaila we decided to revert the change as-is due to concerns of breaking existing code that may be relying on the header not being present.
E.g.

var content = new StringContent("Foo");
content.Headers.Add("Content-Length", "3"); // Throws because the value already exists

Because #16162 mainly impacts diagnostics scenarios (e.g. dumping the current request to the console via ToString), and not the behavior of HttpClient (the header will be sent whether it's initialized early or not), there may still be room for improving those cases by adding more info in HttpRequestMessage.ToString instead of actually modifying the headers collection.

[ManickaP]

Failures unrelated, the originally failing tests are not failing anymore.

[ManickaP]

/ba-g failures unrelated

[michal-zatloukal]

@MihaZupan I strongly disagree that this is only issue for diagnostics. Due to this bug, when I forward HTTP requests on the edge location in our project between our IoT Devices I have to make this:

               private static readonly IReadOnlyList<string> _notForwardedHeaders = new[] { "Host", "Connection", "Keep-Alive", "Upgrade", "Transfer-Encoding" };

...
                var response = await _httpClient.SendAsync(httpRequest, cancellationToken);

                var deviceRequestDuration = Stopwatch.GetElapsedTime(deviceRequestStart);

                var statusCode = (int) response.StatusCode;
                var body = await response.Content.ReadAsByteArrayAsync(cancellationToken);

                var responseHeaders = response.Headers
                    .Where(h => !_notForwardedHeaders.Contains(h.Key))
                    .ToDictionary(h => h.Key, h => string.Join(", ", h.Value));

                // Bug in runtime, 'Content-Length' header not present in the enum, set it manually https://github.com/dotnet/runtime/issues/16162
                if (response.Content.Headers.ContentLength is not null)
                {
                    responseHeaders.Add("Content-Length", response.Content.Headers.ContentLength.Value.ToString());
                }

                // Skip 'Content-Length' anyway for safety (if runtime is fixed)
                foreach (var contentHeader in response.Content.Headers.Where(h => h.Key != "Content-Length"))
                {
                    if (!responseHeaders.TryAdd(contentHeader.Key, string.Join(", ", contentHeader.Value)))
                    {
                        _logger.LogError("Failed to add content header {key} to the response headers.", contentHeader.Key);
                    }
                }

                return new HttpProxyResponse.Successful(statusCode, body, responseHeaders, deviceRequestDuration);

[MihaZupan]

More discussion about this is on #16162