Reverted old code and added simple IgnoreCase to header dictionary

src/Discord.Net.Core/Net/Queue/RequestQueueBucket.cs

@@ -49,9 +49,7 @@ namespace Discord.Net.Queue
 
                 Debug.WriteLine($"[{id}] Sending...");
                 var response = await request.SendAsync().ConfigureAwait(false);
-                string headerDate;
-                bool headerHasDate = response.Headers.TryGetValue("Date", out headerDate);
-                TimeSpan lag = DateTimeOffset.UtcNow - (headerHasDate ? DateTimeOffset.Parse(headerDate) : DateTimeOffset.UtcNow);
+                TimeSpan lag = DateTimeOffset.UtcNow - DateTimeOffset.Parse(response.Headers["Date"]);
                 var info = new RateLimitInfo(response.Headers);
 
                 if (response.StatusCode < (HttpStatusCode)200 || response.StatusCode >= (HttpStatusCode)300)

src/Discord.Net.Core/Net/Rest/DefaultRestClient.cs

@@ -120,7 +120,7 @@ namespace Discord.Net.Rest
                 cancelToken = CancellationTokenSource.CreateLinkedTokenSource(_cancelToken, cancelToken).Token;
                 HttpResponseMessage response = await _client.SendAsync(request, cancelToken).ConfigureAwait(false);
                 
-                var headers = response.Headers.ToDictionary(x => x.Key, x => x.Value.FirstOrDefault());
+                var headers = response.Headers.ToDictionary(x => x.Key, x => x.Value.FirstOrDefault(), StringComparer.OrdinalIgnoreCase);
                 var stream = !headerOnly ? await response.Content.ReadAsStreamAsync().ConfigureAwait(false) : null;
 
                 return new RestResponse(response.StatusCode, headers, stream);