React to IHeaderDictionary changes.

Author: Tratcher

src/Microsoft.AspNet.Server.Kestrel/Http/Frame.cs

@@ -8,6 +8,7 @@
 using System.Text;
 using System.Threading;
 using System.Threading.Tasks;
+using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Server.Kestrel.Infrastructure;
 using Microsoft.Extensions.Logging;
 using Microsoft.Extensions.Primitives;
@@ -53,13 +54,13 @@ public Frame(ConnectionContext context) : base(context)
         public string Path { get; set; }
         public string QueryString { get; set; }
         public string HttpVersion { get; set; }
-        public IDictionary<string, StringValues> RequestHeaders { get; set; }
+        public IHeaderDictionary RequestHeaders { get; set; }
         public MessageBody MessageBody { get; set; }
         public Stream RequestBody { get; set; }
 
         public int StatusCode { get; set; }
         public string ReasonPhrase { get; set; }
-        public IDictionary<string, StringValues> ResponseHeaders { get; set; }
+        public IHeaderDictionary ResponseHeaders { get; set; }
         public Stream ResponseBody { get; set; }
 
         public Stream DuplexStream { get; set; }

src/Microsoft.AspNet.Server.Kestrel/Http/FrameHeaders.cs

@@ -5,16 +5,31 @@
 using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
+using Microsoft.AspNet.Http;
 using Microsoft.Extensions.Primitives;
 
 namespace Microsoft.AspNet.Server.Kestrel.Http
 {
-    public abstract class FrameHeaders : IDictionary<string, StringValues>
+    public abstract class FrameHeaders : IHeaderDictionary
     {
         protected Dictionary<string, StringValues> MaybeUnknown;
 
         protected Dictionary<string, StringValues> Unknown => MaybeUnknown ?? (MaybeUnknown = new Dictionary<string, StringValues>(StringComparer.OrdinalIgnoreCase));
 
+        StringValues IHeaderDictionary.this[string key]
+        {
+            get
+            {
+                StringValues value;
+                TryGetValueFast(key, out value);
+                return value;
+            }
+            set
+            {
+                SetValueFast(key, value);
+            }
+        }
+
         StringValues IDictionary<string, StringValues>.this[string key]
         {
             get

src/Microsoft.AspNet.Server.Kestrel/ServerRequest.cs

@@ -6,6 +6,7 @@
 using System.IO;
 using System.Linq;
 using System.Threading.Tasks;
+using Microsoft.AspNet.Http;
 using Microsoft.AspNet.Http.Features;
 using Microsoft.AspNet.Server.Kestrel.Http;
 using Microsoft.Extensions.Primitives;
@@ -109,7 +110,7 @@ string IHttpRequestFeature.QueryString
             }
         }
 
-        IDictionary<string, StringValues> IHttpRequestFeature.Headers
+        IHeaderDictionary IHttpRequestFeature.Headers
         {
             get
             {
@@ -161,7 +162,7 @@ string IHttpResponseFeature.ReasonPhrase
             }
         }
 
-        IDictionary<string, StringValues> IHttpResponseFeature.Headers
+        IHeaderDictionary IHttpResponseFeature.Headers
         {
             get
             {