Add missing ref-assembly for header propagation

I think this was missed because the PR predates ref assemblies :)

Author: Ryan Nowak

src/Middleware/HeaderPropagation/ref/Microsoft.AspNetCore.HeaderPropagation.csproj

@@ -0,0 +1,12 @@
+<!-- This file is automatically generated. -->
+<Project Sdk="Microsoft.NET.Sdk">
+  <PropertyGroup>
+    <TargetFrameworks>netcoreapp3.0</TargetFrameworks>
+  </PropertyGroup>
+  <ItemGroup Condition="'$(TargetFramework)' == 'netcoreapp3.0'">
+    <Compile Include="Microsoft.AspNetCore.HeaderPropagation.netcoreapp3.0.cs" />
+    <Reference Include="Microsoft.AspNetCore.Http"  />
+    <Reference Include="Microsoft.Extensions.Http"  />
+    <Reference Include="Microsoft.Extensions.DependencyInjection"  />
+  </ItemGroup>
+</Project>

src/Middleware/HeaderPropagation/ref/Microsoft.AspNetCore.HeaderPropagation.netcoreapp3.0.cs

@@ -0,0 +1,52 @@
+// Copyright (c) .NET Foundation. All rights reserved.
+// Licensed under the Apache License, Version 2.0. See License.txt in the project root for license information.
+
+namespace Microsoft.AspNetCore.Builder
+{
+    public static partial class HeaderPropagationApplicationBuilderExtensions
+    {
+        public static Microsoft.AspNetCore.Builder.IApplicationBuilder UseHeaderPropagation(this Microsoft.AspNetCore.Builder.IApplicationBuilder app) { throw null; }
+    }
+}
+namespace Microsoft.AspNetCore.HeaderPropagation
+{
+    public partial class HeaderPropagationEntry
+    {
+        public HeaderPropagationEntry() { }
+        public Microsoft.Extensions.Primitives.StringValues DefaultValue { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
+        public string OutboundHeaderName { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
+        public System.Func<string, Microsoft.AspNetCore.Http.HttpContext, Microsoft.Extensions.Primitives.StringValues> ValueFactory { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
+    }
+    public partial class HeaderPropagationMessageHandler : System.Net.Http.DelegatingHandler
+    {
+        public HeaderPropagationMessageHandler(Microsoft.Extensions.Options.IOptions<Microsoft.AspNetCore.HeaderPropagation.HeaderPropagationOptions> options, Microsoft.AspNetCore.HeaderPropagation.HeaderPropagationValues values) { }
+        protected override System.Threading.Tasks.Task<System.Net.Http.HttpResponseMessage> SendAsync(System.Net.Http.HttpRequestMessage request, System.Threading.CancellationToken cancellationToken) { throw null; }
+    }
+    public partial class HeaderPropagationMiddleware
+    {
+        public HeaderPropagationMiddleware(Microsoft.AspNetCore.Http.RequestDelegate next, Microsoft.Extensions.Options.IOptions<Microsoft.AspNetCore.HeaderPropagation.HeaderPropagationOptions> options, Microsoft.AspNetCore.HeaderPropagation.HeaderPropagationValues values) { }
+        public System.Threading.Tasks.Task Invoke(Microsoft.AspNetCore.Http.HttpContext context) { throw null; }
+    }
+    public partial class HeaderPropagationOptions
+    {
+        public HeaderPropagationOptions() { }
+        public System.Collections.Generic.IDictionary<string, Microsoft.AspNetCore.HeaderPropagation.HeaderPropagationEntry> Headers { [System.Runtime.CompilerServices.CompilerGeneratedAttribute]get { throw null; } [System.Runtime.CompilerServices.CompilerGeneratedAttribute]set { } }
+    }
+    public partial class HeaderPropagationValues
+    {
+        public HeaderPropagationValues() { }
+        public System.Collections.Generic.IDictionary<string, Microsoft.Extensions.Primitives.StringValues> Headers { get { throw null; } }
+    }
+}
+namespace Microsoft.Extensions.DependencyInjection
+{
+    public static partial class HeaderPropagationHttpClientBuilderExtensions
+    {
+        public static Microsoft.Extensions.DependencyInjection.IHttpClientBuilder AddHeaderPropagation(this Microsoft.Extensions.DependencyInjection.IHttpClientBuilder builder) { throw null; }
+    }
+    public static partial class HeaderPropagationServiceCollectionExtensions
+    {
+        public static Microsoft.Extensions.DependencyInjection.IServiceCollection AddHeaderPropagation(this Microsoft.Extensions.DependencyInjection.IServiceCollection services) { throw null; }
+        public static Microsoft.Extensions.DependencyInjection.IServiceCollection AddHeaderPropagation(this Microsoft.Extensions.DependencyInjection.IServiceCollection services, System.Action<Microsoft.AspNetCore.HeaderPropagation.HeaderPropagationOptions> configureOptions) { throw null; }
+    }
+}