Improvements to appending request header

src/Microsoft.AspNet.Server.Kestrel.GeneratedCode/KnownHeaders.cs

@@ -288,6 +288,32 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
             ((ICollection<KeyValuePair<string, string[]>>)MaybeUnknown)?.CopyTo(array, arrayIndex);
         }}
 
+        public void Append(string key, string value)
+        {{
+            switch(key.Length)
+            {{{Each(loop.HeadersByLength, byLength => $@"
+                case {byLength.Key}:
+                    {{{Each(byLength, header => $@"
+                        if (""{header.Name}"".Equals(key, StringComparison.OrdinalIgnoreCase)) 
+                        {{
+                            if ({header.TestBit()})
+                            {{
+                                _{header.Identifier} = AppendValue(_{header.Identifier}, value);
+                            }}
+                            else
+                            {{
+                                {header.SetBit()};
+                                _{header.Identifier} = new[] {{value}};
+                            }}
+                            return;
+                        }}
+                    ")}}}
+                    break;
+            ")}}}
+            string[] existing;
+            Unknown[key] = Unknown.TryGetValue(key, out existing) ? AppendValue(existing, value) : new[] {{value}};
+        }}
+
         public partial struct Enumerator
         {{
             public bool MoveNext()

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

@@ -23,6 +23,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
             Terminated,
         }
 
+        static Encoding _ascii = Encoding.ASCII;
         Mode _mode;
         private bool _resultStarted;
         private bool _responseStarted;
@@ -517,11 +518,11 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
                         ch2 != ' ' &&
                             ch2 != '\t')
                 {
-                    var name = Encoding.ASCII.GetString(remaining.Array, remaining.Offset, colonIndex);
+                    var name = _ascii.GetString(remaining.Array, remaining.Offset, colonIndex);
                     var value = "";
                     if (valueEndIndex != -1)
                     {
-                        value = Encoding.ASCII.GetString(
+                        value = _ascii.GetString(
                             remaining.Array, remaining.Offset + valueStartIndex, valueEndIndex - valueStartIndex);
                     }
                     if (wrappedHeaders)
@@ -565,17 +566,7 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
         private void AddRequestHeader(string name, string value)
         {
-            string[] existing;
+            _requestHeaders.Append(name, value);
-            if (!RequestHeaders.TryGetValue(name, out existing) ||
-                existing == null ||
-                existing.Length == 0)
-            {
-                RequestHeaders[name] = new[] { value };
-            }
-            else
-            {
-                RequestHeaders[name] = existing.Concat(new[] { value }).ToArray();
-            }
         }
     }
 }

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

@@ -11,6 +11,14 @@ namespace Microsoft.AspNet.Server.Kestrel.Http
 
         protected Dictionary<string, string[]> Unknown => MaybeUnknown ?? (MaybeUnknown = new Dictionary<string, string[]>(StringComparer.OrdinalIgnoreCase));
 
+        protected static string[] AppendValue(string[] existing, string append)
+        {
+            var appended = new string[existing.Length + 1];
+            Array.Copy(existing, appended, existing.Length);
+            appended[existing.Length] = append;
+            return appended;
+        }
+
         protected virtual int GetCountFast()
         { throw new NotImplementedException(); }