Fix HttpHeaders Keys case sensitivity (#50606)

src/Servers/Kestrel/Core/src/Internal/Http/HttpHeaders.cs

@@ -115,7 +115,7 @@ internal abstract partial class HttpHeaders : IHeaderDictionary
 
     bool ICollection<KeyValuePair<string, StringValues>>.IsReadOnly => _isReadOnly;
 
-    ICollection<string> IDictionary<string, StringValues>.Keys => ((IDictionary<string, StringValues>)this).Select(pair => pair.Key).ToList();
+    ICollection<string> IDictionary<string, StringValues>.Keys => ((IDictionary<string, StringValues>)this).Select(pair => pair.Key).ToHashSet(StringComparer.OrdinalIgnoreCase);
 
     ICollection<StringValues> IDictionary<string, StringValues>.Values => ((IDictionary<string, StringValues>)this).Select(pair => pair.Value).ToList();
 

src/Servers/Kestrel/Core/test/HttpHeadersTests.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using Microsoft.AspNetCore.Http;
 using Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http;
 using Microsoft.Extensions.Primitives;
 using Microsoft.Net.Http.Headers;
@@ -298,4 +299,12 @@ public class HttpHeadersTests
         Assert.Null(httpHeaders.ContentLength);
         Assert.False(httpHeaders.ContentLength.HasValue);
     }
+
+    [Fact]
+    public void KeysCompareShouldBeCaseInsensitive()
+    {
+        var httpHeaders = (IHeaderDictionary)new HttpRequestHeaders();
+        httpHeaders["Cache-Control"] = "no-cache";
+        Assert.True(httpHeaders.Keys.Contains("cache-control"));
+    }
 }